It sounds to me like that double pole breaker (rear wall) that you assume is for the "water heater", is really the "main disconnect", and the single pole breaker below that is for the "water heater". RV water heaters are generally not 240V (i.e.: double pole breaker). That may be where the problem is. This "main disconnect" box probably feeds the other "distribution" box (at the door) you mentioned.
In any event, you need to get your meter out and check for 120V power incoming into both breaker boxes by taking the cover off the boxes and checking directly on the incoming wires. And while you got the cover off, you can also check for power on the back side of each circuit breaker in the boxes. You need to isolate where "power is" and where it "isn't" to the shortest possible stretch of real estate; and go from there.
This all assumes the solar system you have is solely tied into the 12V side of things (you did not mention an inverter) and not the 120V. If it's tied into the 120V system, then you have to figure out where, how, and if a transfer switch is involved. All this is a whole new ball game.
