I think I'm the king of the boiled batteries. With two boats and a camper, I seem to go through these things like most people go through AA's.
I have managed to keep the same set of batteries in the camper for the last couple of years.
Basically, the first step was to get rid of the electro-mechanical magnatek converter that was in the camper. If you leave that connected, you will boil the batteries out. I replaced mine with one of the Intellipower
units and a charge wizard. I think it was the 30 amp model, about 150$. This helps a lot.
The other thing was to get the (Flow-Rite
) battery filler device from CW . Basically, it replaces the battery caps and has a tube and a pump you use to automatically fill the battery to the correct level without removing it from the camper.
Doing both of these is a little spendy, but I figure it's better than going through a set of batteries every year
Not trying to help out camping world here or anything, just saying it's what I've done and it helps!
Oh, and to actually answer the question you asked, about once a month, but with my setup, it's more a matter of just topping off the batteries...