If it is a Dodge chassis The gravity fill will be in the latch side / RS post on the rear door. The fresh water drain cap is behind the left rear wheel. You may have to lay down on a cardboard or similar to get to it. The dump /Tee handles for the black and gray water are located under the driver side running board.
The gravity fill hose may be bad or if the tank is full, like was said in a previous post, the water will run out of the top vent like it is raining. This is by design so as not to over fill. Do not trust the gauges!!!!
Hint, I carry one of the silver bubble wrap windshield covers to lay down on to check anything underneath. It is light, soft and folds up so it does not take up much room.
