Black/Grey/Fresh water issues. If you flush with Grey water, you may not have enough grey water, when it is time to dump the tanks. Dump the black first, and then dump the grey water. The grey water will flush any residues out of the valve and hose, that were left from the black side.
If you do set up flush with grey water, you may need to run extra fresh water into the grey tank, just prior to dumping.
I don't know where your equipment is located, but my van has the propane tank mounted between the frame rails, near the rear bumper. The fill connector and shutoff valve are behind the license plate location.
There must be some place to mount the propane tank, that is protected and allows access to the connectors and valves.
Where it is now is asking for trouble.