motorhome?

This is a quess as only had caravans but had similar on my Lunar Solar caravan.

Whale Water pressure switch in the cold water feed pipe
it's white about 2-3 ins high and about 2.5 long thumb screw on top and that's turned clockwise to close completely. Open a cold tap then turn the pressure control thumb screw anit-clockwise slowly until it starts to click leave it and turn off the tap.
Turn tap back on see if it has stopped the pulsating and turn tap off to see if that cures the 'run on ' of the pump ie: running on without emitting water.
Sorry can't be more helpful but as explained I never had motorhome and set up may be different.

type in search engine box Whale water pressure switch.

Bob
January 2011