As mentioned by Terri G, don't buy a "starter" battery rated in only CCA/Cold Cranking Amps. Look for deep cycle/marine batteries that have an AH/Amp Hours rating. (Some batteries are "hybrid" batteries designed for both deep cycle use and for starting a boat motor, so some batteries will have both ratings.)
Amp-hour ratings for the most affordable classes of batteries tend to be 55, 80, and 120 Amp-hours. Larger batteries are available, but not only are they much more expensive, they're very, very heavy. If you need the extra capacity it's cheaper and easier on the back to get a pair of 80 or 120 AH batteries than it is to pay for and heft one of the larger units.
Two main types of batteries: "Wet" batteries that have caps on top of the power cells so you can open them up to add distilled water. Wet cells are cheaper, but the reason they have those caps is because you need to open them up periodically and check, then fill the battery. Its best done once a week or more while you're out camping and at the beginning and end of each trip.
There are also sealed, maintenance-free batteries. There are two types, AGM/Absorbed Glass Mat batteries and "Gel Cell" units. The gell cell style of maintaiance-free batteries have very specific charging protocols that most charging systems don't follow, so they do not last as long as either wet or AGM units.
AGM batteries are more expensive, but last longer than wet-cell units, waste less energy off as heat as they are charged, and hold their charge longer. If you plan on installing solar
panels, AGM batteries are a good first choice, if you can afford them.
Most modern battery chargers and trailer converters have a peak charging voltage of 14.5 volts or so, which is safe for AGM batteries. If, however, you have a trailer with a charger or converter built prior to 1990 or so, you might want to check its charging voltage before buying an AGM battery. Many older charger/converter units charge at 17 volts, sometimes more. These higher voltages are damaging to any deep-cycle battery, but are particularly hard on AGM units.
Deep-cycle, wet-cell batteries have a typical lifetime of two to three years if they are maintained properly. Walmart sells them with a two-year warranty for under $100, and if the battery fails within the two-year period they exchange it for a brand-new unit free of charge, even if they fail because you forgot to maintain them properly or have a 17-volt charger.
Maintenance-free AGM batteries last 2-3 times as long, sometimes more, but cost 2-3 timeas wht a wet-cell unit costs. (My AGM unit has lasted five years, but cost 3 times as much as a wet-cell battery.)
One last piece of information: a good investment for your battery is to not use yur converter to keep your battery charged while your trailer is being stored. It's much better to get a battery maintainer, and even better to get a BatteryMinder maintainer (about US$60), which will extend your battery life by forcing the battery's chemistry to remain activated in the charged state.