Hi Dylan,
Is the coach level side-to-side? If the door-side is a little low, water will accumulate on that side of the tank whereas the drain is on the off-door-side.
Even if the coach is level, the tank may not be. If the supports have come loose from the frame on the door-side, the tank will be off level.
Whether an issue with the valve, or the tank position, to investigate, you'll probably have to drop the front section of the coroplast, at least in part.