one problem I see is when you are talking about the strength of pex. The failure point is a little more difficult than you made it sound as it isn't based solay on pressure, but rather a factor of temperature and pressure. Also, it depends on the type of PEX you get, yes there are different grades, some that will hold more pressure but are very stiff and some that are softer, so they bend easily but don't hold as much pressure. then there are simply inferior brands.
for instance, 5 years after doing renos I would start having mysterious failures in my pex hot ware line. just tiny splits that let a mist come out, and each time I would cut out a section and replace it. They were starting to show up on a regular basis and I had my plumber involved then the manufacture of the pex got involved. They sent out some equipment to be installed that monitored pressure and temprature over two weeks and what we found is the city turned up the pressure to 95psi and it quite frequently spiked to 120psi and I didn't have a pressure regulator. So, this issue was only happening on the hot water side, and I have a ring main on demand hot water set up that I was running at 135 degrees. So, what I found out is the hotter the temp the lower psi the pex can handle and a combination of 135 degrees and spikes of up to 120psi was enough to make failures.
Given the lower pressures we run in rv's we would never see this issue but be careful even going to 60psi. Yes, the pex itself may handle it but the problem with the rv industry is generally they use cheap plastic crimp fitting and tap sets which will fail long before the pex itself. the line won't split from freezing it's the fittings and fixtures that break now.
