On an even smaller trailer ( an X-139), I use two group 31s by NAPA. One battery is in use, and the other is a spare. Each is rated at 110 amp hours.
But honestly, that may be overkill for most folks. We do a lot of off-grid boondocking in cold weather, so we need a lot of juice for the furnace and the water heater, even with a solar panel.
2012 Fun Finder X-139 "Boondock Style" (axle-flipped and extra insulation)
2013 Toyota Tacoma Off-Road (semi-beefy tires and components)