[Festus2]

Having 2 batteries may not be necessary if you only dry camp once or perhaps twice a year. You didn't mention how long your stays are without power but you should be able to get 3-4 days in with one battery providing you are mindful of what you use and for how long.
I'd suggest that you look into replacing some or all of your incandescent interior lights with LED's. These will draw far less on your battery -- about 1/9 to 1/10 less - and your battery will last considerably longer with that saving.