Re: Travel Trailer Batteries
When the battery is being charged, some of the liquid is 'boiled off' as gases (including Hydrogen, so make sure the compartment is well vented).
While plugged in, you need to keep the batteries topped off with (distilled) water. The easiest way to do this is with a built in battery filler (don't recall the brand, but Camping World carries them). On mine, I just stick a hose in a bottle of distilled water, and squeeze the bulb until it doesn't squeeze any more. Batteries all (correctly) filled!
When I was looking at a solar system many years ago, I saw a gadget which replaced the battery cap and was supposed to capture the gasses and reconstitute the water back into the cell. Don't know if these would fit, or how they would work, or even if they are still available, but you might take a look for them.
Another option would be to investigate a trickle charger (perhaps solar) for when the unit is in storage. I got a plug in battery maintainer from Camping World, but haven't tried it yet to see if it minimizes water loss. I'm hoping that a trickle charge does not 'gas' as much as a regular charge.