To understand your choice, you could go here:
http://www.phrannie.org/battery.html
but if you want a quick recommendation for an inexpensive set-up that will allow the occasional night of boondocking, buy two group 27 "marine/deep cycling" batteries from Wal-Mart. (note the inverted commas) You might like to look on the tops and get two from the same batch number. I also take my digital voltmeter, and choose the two nearest in voltage from the same batch number, but you don't need to bother with that. If you also plug them into a "battery minder" when away from the trailer for more than a few weeks, they should prove an economical solution. They work for me. Nick.