Yeah, they are a real PITA to access. Behind the propane tanks, remove some straps in a space barely large enough for my fingers to fit, snap open the box lids with a long screwdriver, just to see the top of the batteries.
I try to check them at every propane refill, but sometimes I say heck with it and I'll check them later. Then usually don't.
During our last trip I noticed they went from full charge to 50% in one night with the heater on. So I checked and they were thirsty. They are four year old batteries, so it may be time to replace.