Randy,
When I got my coach it was pretty much patched together with some of the manifold valves in place and some of them bypassed or plumbed in with other valves.
Instead of repairing what I had I bought 2 pex copper pass though manifolds and mounted them vertically. I put additional 1/4 turn ball valves on the bottom for extra low point drains, and plumbed the original lines up to the manifolds pex valves.
Now I can turn any lines off just like before and I have not had any leaks in five plus years. It wasn't super cheap, but neither are those plastic fittings. It will also be easy to repair if something does fail.
I just mention this in case you want to go in a different direction. I have put the pex manifold systems in two of my homes and not had any issues in over 15 years.
Hopefully you will find this helpful.
