I haven't heard of that water heater problem before, it's not normal but maybe in places with 'sulfur water' it might be.
The battery is probably old and weak and since this is a rental it's probably mistreated too. There is a skill involved in electric management for boondocking. One you'll pick up but basically you have to be paranoid about what you run electrically on the battery. Boondockers also generally have at least two big deep cycle batteries if not golf cart batteries. Running something like a microwave will kill the battery quickly.
The boondocking forum may have some advice for you.