[RMcNeal]

In past trips during cooler weather, we were able to go 5-6 hours without running the fridge on inverter (mostly because the inverter and disconnect were bad and had to wait on replacement). Everything stayed cold as long as we had it cold before leaving. Now with warmer weather, I've noticed that even on a 4 hour trip the fridge would be up to 47-48 degrees when we did get hooked up again. Freezer in the 20's. They were 36 and -2 degrees when hooked up.
I barely have room for 2 batteries without completely redoing the battery compartment like lightsout suggests. I would consider the 6v, but if 1 goes, I'm dead in the water. At least with 2-12v, I can still limp home and get a replacement. I have never owned a deep cycle lead acid battery. How often must you check/add water? Is it a weekly thing or maybe every month or so? I have only had maintenance free, I assume AGM gel mat batteries. So never had to add water or anything.
Any advice regarding 2-12v other than making sure they are matched and same age/size, etc?