I've heard of troubles with the plastic PEX fittings. At least in early
home installations. It seems they weren't very sturdy and suffered
reliability problems. In a trailer, I think these problems would be
amplified. Even if those early bugs have been worked out, I still
wouldn't trust a plastic shut-off valve to be long lasting.
The copper fittings for PEX that crimp on seems like a better way
to go, IMO. But, I have no personal experience here and would
love to hear from any folks that have. New plumbing is next on my
list if I ever get my floor closed back up.
Jimbo / '76 Sovereign