Boats basically always have the pump out outlet way above the tanks (it's up on deck whereas the tanks are down low in the boat). However boat pump out stations are set up to... pump. In other words, they basically vacuum the cr-p up and out. But of course RV dump stations are based on gravity (unless one does the pumping/macerating setup onboard the RV).
Looks like you know your way around customizing
BG
PS: But then of course there is everything else that's basically at the same level: The rear steps, the trailer hitch (which is what I drag most often), and etc. So perhaps with a 6" lift, there is no point to moving the dump valves up, since you'd still have everything else that was at that level (and all of it will be 6" higher already).
