Lots of question to help you diagnose. Is your water pump on? It won't matter if you turn off city water if the pump is on you will still have pressure. Have you used the RV since winter? Start checking pipes in the bathroom and your outdoor kitchen. Even if you've never used them, pressured water will find it's way there. Pex is pretty resilient to cracks so I would check fittings on fixtures, the faucet housings, unions and connections etc. The dealer should certainly be willing to take a look at it. This is a brand new unit right?