What is the maximum water pressure ?

The max pressure will depend on the materials the pipes are made of.

We have Pex tubing in our pipes and we set our regulator at 45 p.s.i.

The question can't be answered accurately. As Dog Folks posted, it's dependent on the pipe material but also involves fittings, clamps, and the state of all involved.

FWIW, I have a brand new PEX installation with valves rated at min. 150 PSI. I have a steel pressure tank with all fittings made of metal, including braided steel supply lines. I've had the system filled at 80 PSI for days and haven't had any issues.

If you're using a regulator is there an issue with how the water is supplied through the system?

An RV should be able to take 50#, but it is an RV, so who knows?

The above answers are correct. It depends on what material was used. The system is only as good as the weakest link. RVs aren't known for using top quality materials. I would never run a RV without a "QUALITY" regulator set at no higher than 50#. Many are factory set at 45#. That is the setting that I use. The cheap inline regulators are an accident waiting to happen. It is cheaper to pay the difference up front and not have to deal with damaging your system, especially while you are on the road. The last RV that I rebuilt will easily take over 100#, but why subject it to that.

I believe the factory, when I had some warranty work done, tested the plumbing at 80psi. I don't think I would want to run it at that pressure though. I have PEX plumbing.

If you have a preset regulator it is set for 45#
If you have an adjustable regulator set it for 50# static (not flow)

PEX pipe can handle 150# but the fittings ????

I have Pex tubing and I run it around 50-55 with a Watts regulator but I have been known to crank it up to 70-80 when showering.

PEX tubing is available in different pressure ratings and RVs typically use the one rated for max. 100 psi. You will find the rating on the wall of the tubing. I just did some plumbing mods and all the tubing was marked 100 psi. I have seen the same on two previous TTs as well. I bought some red and blue PEX from a local RV dealer and it was rated 160 psi.

Fittings (tees, elbows, couplings, etc.) are usually plastic and I have not yet seen a rating for them. Things like check valves, toilet valves and faucets probably aren't good for higher than 100 psi, but that's just a guess on my part. I don't think I'd ever want to exceed 100 psi. We have a Watts 263A inside our TT set at 50 psi. We also use a high pressure Valterra hose rated for 200 psi.