As part of our routine maintenance, we check the water level of our RV Batteries every 3 months.
According to Interstate Batteries, while RV and Marine batteries are considered to be maintenance free under normal operating conditions, the batteries may need to be checked for water loss if they are overcharged or overheated. While we check ours every 3 months, if you live in a very hot area you may want to check them more often.
To check the water level of your batteries, you will remove the vent caps. You can either slide the lock (like our house batteries) or you can pry them off with a screw driver (like our chassis battery). Look down into each well to make sure that the water is covering the lead plates.
If the water is low you will add DISTILLED WATER up to the bottom of the tubes that go down into the battery. You want to make sure that the water is always covering the lead plates, otherwise you can damage them. You also want to be sure to not overfill them because it could result in damage around the battery from acid overflow.
Periodically you should also clean the battery terminals on the top of the battery. They can be cleaned with a mix of baking soda and a wire brush. Once you scrub them clean, you can rinse them with clean water and coat them with a commercially available terminal sealer or high temperature grease.