Also, Shower heads are fitted now with Water Saver fittings, all they are is a plastic button with holes which restricts flow , great theory if you have fire mains water pressure but in a van all they do is reduce poor pressure even more, I pull the things out, you will find it fitted at the end of the shower head where the hose connects or where the hose connects to the wall outlet, just flick it out with a small screw driver, I have done the same with the ones in my house as we don't have great pressure, you will like the improvement, they can also act as a screen for small bits of stuff and block up, you may have to replace with a rubber washer if it doesn't seal but not usually.
They can look like this, various colours and shapes.
...................with the PLV I just pulled the moving bits out and threw them in the bin and put the valve back on, have neverhad any drama since.....if pressure too high the hose fittings will pop off on the hose anyway and the Guest water lines and fittings are rated for 150psi so should only pop off if fitted wrong.