When filling with water, most of it dumps out the side.

We used to have a 1990 Class C Warrior and had the same problem. Water tank was under the rear corner bed. Vent seemed fine. So we took the mattress off, lifted up the wood under the bed to look at the tank and fill line. The fill line was laid in there and affixed with a kink or bend that was not conducive to free flow. Once we fixed that line, the job of filling water went much better.