[DebNJim B]

The pex lines in an RV should all be 1/2" and at 45-60 psi there should be no issue with pressure drops if two faucets are use at the same time.
The op's issue sounds to me like a restriction has developed somewhere. First check the screen on the inlet connection. Also if you are using an inline filter, it could be partially plugged. At my permanent site I have learned (the hard way) that if the spigot has been shut off for a couple of months rust will show up. So now I hook up a hose and flush it out before connecting up. You'd be surprised how just a little bit of rust particles can cause a huge restriction in the flow.