[JRTJH]

There are two types of black tank nozzles. One is a stationary nozzle with several holes that spray water in a random pattern. The other is a rotary nozzle with two/four holes (depending on model). That type spins and sprays water in a circular pattern. Either one is effective to cover the inside of the tank. The rotary nozzle needs water pressure around 40 PSI to operate effectively, but any significantly higher pressure runs the risk of "overspeeding" the nozzle and damaging it. I've seen several that were "twisted off" or "blown off" of the fixture. I would guess (although I've never seen it) that the ones with stationary nozzles would also restrict water flow through those small holes and eventually build up enough pressure to potentially cause damage.

Somewhere a few years ago I was reading the installation instructions on an "add on" black tank flush in a Camping World. It was the type with the rotary nozzle, clear hose and "under rail" connection for your garden hose. The operating instructions indicated that the unit was designed to operate at a minimum of 40 PSI and cautioned not to exceed 60 PSI.

So, I've always used my water regulator with the black tank flush. I've seen campground water supplies that exceed 100 PSI at the "low pressure sites" and upwards of 120 PSI at those closer to the street where the campground tied into the city water supply.