Patrick -
I carry a gas powered pressure washer as well, don't think gravity will work. You can easily tie off of the fresh water drain, but it is a small pipe and it takes about 2 hours plus to drain my water tank. (120 gallons I beleive)
I have a outside shower which might work, just need to look at a way to exchange shower head and fitting for pressure washer. I know my water pump can take it (changed the stock whisperking for a
Flojet Sensor Pump which my wife let run dry for 5 hours and it still works. This pump is super quiet and only runs at the speed need to supply water requirment. Versus the on/off of the whisperking.
As of yet I have not needed to use the on board water to run the pressure washer, I am sure you will when your out in the boonies.
2003 28' Show Hauler Motorhome on a 1995 FL 120
www.showhauler.com