How to replace/re-engineer leaking fresh water drains

If those plastic valves you linked are meant for PEX, you should be OK. Note that they have a special note on the angle drain valve, saying the flow has to be in what looks like the opposite direction from what I think you'd like.

The brass valves in our coach look like Some are straight as shown in the pic, a couple others are 90* angled. They have to be crimped, but they work well after all these years.