Some parks do suffer from low pressure, but in the main, pressure at most parks is more than adequate, easy to check just turn on the tap before connecting, if there is a good rate of flow from the tap the problem is somewhere in the plumbing in your van, there may be a pressure regulator in your system that is either faulty or is placing too much reduction or it may have a small mesh filter that has become blocked.
Just work your way thru the plumbing disconnecting and checking the flow at each disconnected point can't see how you can void any warranties by disconnecting fittings, but would look for the pressure reducer first and check flow after reducer.
Cheers
David

David is so right in what he says, I have had the same problem and found it was a very small mesh filter on the fill side, hope that helps.