Our PUP ('88 StarCraft) had a small freshwater tank, and the check valve kept failing (and was a HUGE pain to replace, due to its location WAYYYY under the sink with only a tiny access door), so we eventually gave up & just used thermoses with little spigots on them as our sink water. Many PUP sink faucets require you to unscrew them partway and then pump like mad to get water from the freshwater tank.
As for gray water - most PUPs of our era, and probably most now, don't have any wastewater tanks, but instead there is a drain either under or on the side near the bottom of the freshwater tank. We drained into a bucket and dumped the bucket when it got full. Most CGs require that to be dumped at the dump station, though we sometimes would use the closest CG toilet.