Hi Mabear,
Welcome to the Heartland Owners Forum.
Heartland makes 5th wheels and travel trailers, but no motorhomes. So the advice you might get on this forum is probably going to be generic; it may or may not apply to a motorhome.
In general, the black tank flush has an Anti-Siphon Valve so that if the city water pressure ever goes negative while using the black tank flush, the water supply is not contaminated. You could have one of those devices under/behind a cabinet or bathroom wall. If you don't use a water pressure regulator, and go to a park that supplies water at 100-200 psi, you could blow out the plastic Anti-Siphon Valve and spray city water all over the place.