There's no free lunch when it comes to batteries. You really need to consider how you use 12v power in your camper to determine what battery system makes sense for you.
Automotive starting batteries are designed to deliver large amp loads for short duration like when you're starting your car. You can kill one in fairly short order by discharging them with light
loads over a long period and then recharging them. They are designed to deliver a quick short punch and then be recharged immediately.
Deep cycle batteries are designed to be discharged slowly over a longer period (say overnight) and then recharged with a proper recharging cycle. The chargers for deep cycle batteries typically are 3 stage types delivering a large ampere charge initially (bulk charging) then tapering off the charge to a low amp trickle (absorption) and then finally dropping voltage and amps to a very light
maintenance charge (float). If wet cell deep cycle batteries are discharged repeatedly to no more than 50% of capacity and then properly recharged (and electrolyte level maintained) they can last 10 years or more.
Other battery types such as gel cell or AGM have different recharge cycles but basically behave like deep cycle batteries although they can also serve well in starting type applications.
Wet Cell Batteries deliver 2.12 volts per cell no matter what brand or type. A 6V battery has 3 cells (connected in series) and fully charged and rested delivers 6.36 volts. Likewise a 12v battery has 6 cells and should read 12.72 volts when fully charged and rested (rested means no load or charge activity for 24 hours) Gel cells and AGMs give different rested readings.
If you connect 2-6volt batteries in series, you get 12 volts (nominally) because you now have 6 cells connected in series. Suppose that the 6v batteries each have a capacity of 220 amp hours; when connected in series the pair of 6v batteries together deliver 220 amp hours and 12 volts. The voltage doubles but the capacity does not.
Similar sized 12v batteries (already has 6 cells) deliver only about 110 amp hours because the cells are physically smaller however when connected in parallel to another 12v battery the voltage stays the same (12v) but the pair together deliver 220 amp hours. In this case parallel connection doubles the capacity but not the voltage.
Connecting two 6v batteries in series or two 12v batteries in parallel yields about the same amount of power for roughly the same amount of space and weight.
That said, golf cart batteries (6v) are so common now that they are really cheap. A top of the line Trojan or Rolls 12v deep cycle battery of say group 31 size will probably set you back around $250 so maybe $500 for a pair. At Sam's Club you can buy a pair of cheap golf cart batteries for about $100. With proper charging and maintenance the Trojans will probably make it 10 years but the golf cart batteries will probably go at least 5 years and maybe more, again provided that you care for them properly.
Over the 10 years, even if you replace the golf cart batteries 4 times, you will still come out cheaper than the expensive top of the line wet cells.
On the other hand, you must be diligent about maintenance or you won't get close to that life.
If you're like me and don't want to mess around with topping off water and cleaning terminals and you occasionally forget to recharge when you should then maybe you should consider gel cells or AGM batteries. These batteries are sealed and require no maintenance other than a proper charging voltage. If you let them run down below 50% charge, they will usually charge right back up. They will also accept a faster charge than wet cells with acceptance rates up around 50% of capacity until they are almost fully charged. However they will not tolerate charging voltages above 14.1 volts. Above 14.1 volts the electrolyte will boil away and since you can't replenish it by adding water, the battery dies.
These types also have the advantage of being much more shock resistant and of course they don't spill. AGMs can be mounted in any position including upside down so that may be an advantage for some. Gels and AGMs also don't out-gas (unless boiled over) so terminal corrosion is not as common as it is with wet cells. All that said, if your batteries are located in a hard to service spot, or you tend to abuse your batteries, you may be ahead with one of these types. They tend to be more expensive to purchase but over the long haul they come out cheaper than the Trojans or Rolls wet cells although not as cheap as golf cart batteries. I have three gel cells in my boat, 2 -12v group 31s and a group 24 starting battery that were installed in 2000 and still seem to be going quite strong.
Whew, I didn't mean for this to be so long ... and we didn't get to the good part yet.
To figure out how many amp hours of battery you need, you should add up the power requirements of all the 12v appliances you will use between charging cycles and consider this to be about half of the minimum capacity you will need. In other words if your consumption between charging cycles is 50 amp hours then a battery bank of 100 amp hours or slightly more would represent the minimum. Adding more battery than roughly twice your needs will only increase the charging requirements. But remember that you don't want to discharge any battery to more than 50% of capacity.
On my boat for example, the two group 31 gel cells have a capacity (or had when new) of a total of 190 amp hours. Generally, over night we consume about 60 amp hours. During the day (on mostly sunny days that is) our solar panel
provides enough energy to break even so that means in 24 hours we have to make up that 60 amp hour deficit. We accomplish that by running the engine and charging from a heavy duty alternator coupled with a 3 stage regulator set-up for gel cells. It takes about 2 hours engine time each day to keep up with our needs. Most of the 60 amp hour deficit is due to the 12v refrigeration system, maybe 10 amp hours or less is lights
, computer, radios, etc.
Since we run to less than 50% of our house bank capacity, there isn't much to be gained by adding more batteries. 60 amp hours a day is 60 amp hours a day period. Sure, with a larger bank say double the size (say 380 amp hours), I could go two days between charging cycles instead but then I would have to make up double the deficit (120 amp hours roughly) by running the engine ... maybe 4 hours running time without changing the charging system.
Back to campers. Our requirements are likely different than yours and somewhat unusual so let me explain the requirements first and then tell you what I have in terms of batteries and charging systems.
We are based in Florida. We're not here in the winter so our camper traveling is over the summer and usually to the Berkshires in Massachusetts and places along the way. We need air conditioning
. This means we need to plug in. No battery system will provide that kind of power. We also have an all electric camper. No propane
except for a gas grill and camp stove that run on small bottles. We run a 120v dorm room size fridge
off the battery and a 1500 watt inverter and charge the camper battery from the tug when under way. We switch the fridge
to 'shore' power when we park and I carry a small 120v charger just in case but I've never had to use it. Since we're plugged in anyway, we use our microwave
a lot and we have used a small electric heater from time to time as well as the water heater. The rest of our battery requirements are the lights, and if there's no water hook-up, the water pump. We also have a small (1000 watt) gasoline generator
that could be used when we can't plug in.
We meet our requirements just fine on a single 12v deep cycle battery (it's what we had handy at the time). Mainly though the battery is just running lights and serving as ballast for the fridge system.
Obviously if you're out boon docking and seldom plug in then your requirements will be different.
Be happy to answer questions, sorry it's so long.