First, if you're going to install a solar
panel, you should plan on making sure your daily electric consumption is less than your solar
panel's energy production. For this it's good to know the math.
So, how much solar power
does your panel produce on a "perfect" day?
First, on a perfect day with perfect solar
energy generating conditions and perfect alignment of your panel to the sun's incoming light
, your panel can olny generate about 67 Amp Hours of power. (100w / 12v = 8.33 Amps/hour. 8.33 Amps * 8 hours = 66.66 Amp Hours.)
Alas, your solar system will almost never operate at 100%, so, let's reduce that output by about third, to 45 Amp Hours per perfect day. This is the maximum number of Amp Hours in your daily energy consumption budget, but be aware that there are many days when your solar panels won't achieve this goal. Like when you park in a forest or during the winter time, when there is much less sunlight. You might want to budget for just 22.5 Amp Hours of energy use, just to be safe.
Next, let's consider your battery. It's good to be able to carry at least one full day's sunlight in your battery. It's also preferable to make sure your battery doesn't fall
below 40% charge. So, if you only ought to use, at most, 60% of your battery's charge and you want to carry a full day's worth of sun in your battery, that means you should have a minimum 75 Amp Hour battery. (45 Amp Hours / 60% = 75 Amp Hours.)
You might want to store some multiple of a day's sunlight, particularly if you like to park your solar panels in un-sunny locations. (Many National Forest locations come to mind.) The next larger battery sizes are 100 and 120 Amp Hours. If you have a daily energy use of 22.5 Amp Hours a day (which I think of as a luxurious amount of power), you could camp for more than 3 days on the energy stores in a fully-charged 120 Amp Hour battery.
My next thought on batteries is there are really just two ways to go: Really expensive or really cheap. A really expensive battery will never let you down and can last a decade if you keep it on a BatterMinder trickle charger/desulphinator when you're not using your trailer. A cheap battery will last you a couple years, maybe less.
My choice for an expensive battery would be a group 24 or 26 Trojan AGM battery with a Battery Minder trickle charger. They cost around $200, but they're maintenance-free batteries and they last pretty much forever if you keep them on a desulphinator when you're not using them.
For cheap, I'd go with a Walmart Marine batteries. They cost $80-100 and have one or two year, full-replacement warranties, and you can find a Walmart just about anywhere to get a replacement. The downside is you *will* need to replace them every few years, and they'll hold less and less charge over time.