The blue and red RV PEX is rated 100 psi and you'll find that printed on the exterior if you look closely. If you buy the red/blue stuff at a hardware store or RV shop, then chances are it's 160 psi. I have not been able to find out what the plastic fittings are rated for though. I suspect they're rated to match the Pex, 100 psi. There's a lot of RVers not using regulators, and as said, many CGs get up to 100 psi and sometimes a lot more. If fittings couldn't withstand high pressure, I'd expect a lot of reports of failed fittings out there.
50 psi is nothing for an RV system. We have our Watts regulator set to 60 psi and that's actually what the RVwaterfilterstore.com guy recommended.
PEX can easily withstand freeze thaw cycles and I've had a charged line in an unheated area in our garage year round for 8+ years and it does fine. If anything would fail in from water freezing in an RV, I would guess it would be plastic PEX fittings.
I blow our lines out at 50 psi. Easy, quick and no antifreeze to buy. DW refuses to allow anything other than water into the piping. What I don't read much about are the two types of anti-freeze people. The cheaper type has ethanol and can cause a bad taste and odor, can damage seals and can be difficult to get rid of the taste and odor in the spring. The better stuff has propylene glycol and doesn't have the problem with taste and odor or damaging anything. If you are using antifreeze, spend a buck or two more and get the good stuff. Both are readily available and Camco makes both.
