Seelevel II 709-P3w model. Went to check all 5 functions, battery, fresh water, gray, black and Propane gas levels and fresh water had an error code and now shows zero when I know its at least 1/2 full. Any ideas where to start. Without a visual on the tank, I can't see mine, its covered up so I really need it. It was installed when I bought it. I know there is...