The "old grey stuff" is PVC. It was prone to rupture under high pressure when heated. But, because it is polyvinyl chloride (PVC) it also imparts a taste to water it contains ("outgassing"). Trailer pressures shouldn't be a problem. And be sure to flush the lines out a bit before using it for drinking. But it's still PVC, and for some, that's not a desirable product!
PEX, on the other hand, is "Crosslinked Polyethylene" (PE) (X). Polyethylene is inert once hardened. It doesn't outgas, and it is very tough. There is also Blue PEX, which has an aluminum tube sandwiched into it, which allows bending of tight curves (Ideal for campers!). Be sure to use a bending sleeve to prevent kinking.
These products can be used with slip-on fittings, which eliminate the need for a crimping tool. I used these with PEX in my Boler
re-plumb. There might be concern about vibration loosening such fittings, so make sure the pipe is fastened down. As Kevin says, a crimper is easy to find, and cheap by the hour (Prepare all the pipe and fittings first, then crimp all at once).