I was looking at a similar issue. I'm also wanting to go dry camping but only had the one deep cycle battery. I looked up the battery specs and found that it had a reserve capacity of 160. Unfortunately, that didn't tell me the amp hours so I found this:
1.Multiply the reserve capacity by 60 to convert it to seconds. For example, if a battery offers a 100-minute capacity: 100 x 60 = 6,000 seconds.
2.Multiply this length of time by 25, which is the battery's amperage. Example: 6,000 x 25 = 150,000. This is the number of Coulombs of charge in the battery.
3. Divide this answer by 3,600, which is the number of Coulombs in an amp-hour. Example: 150,000 ÷ 3,600 = 41.67. This is the number of amp-hours in the battery.
Once I had the amp hours I looked for some info on what amp hours are. This site (http://overlandresource.com/what-is-...ttery-capacity
) has a good explanation of amp hours and battery draw.
Based on the lame batteries that came with the trailer, I decided to add another 12v in parallel, get a little Harbor Freight generator to charge them each day, and get a Heater Buddy as a contingency.
Best of luck!