Originally Posted by bubbles
The way to clean out the black holding tank is to dump then close the valve and allow the cleanout input (water) to almost fill the tank (watch your level indice for the amount in the tank). This will take sometime because the spray nozzle in the tank typically has small holes. Open the valve with the input still flowing and if you have a clear adapter at the discharge you will see if the procedure should be done again. If not then a couple times of refilling and draining will probably sufice. The black tank is VENTED TO THE OUTSIDE and therefore no tank expansion will occur. There will be no damage to the tank. Typically, when you are finished and have closed the valve for the last time and shut the water off and disconnect the hose there will be residual water in the line from the input side of your unit backflow preventer. This water is not coming from the black tank. This water is from the closing of the backflow preventer. Another thing, your backflow preventer is typically mounted high above the black tank and this perhaps would suggest the backflow preventer may not even be needed.
Caveat Lector in this case. There are warnings on the camper itself (stickers) and the truboflush manuals not to use the system with the dump valve closed. You, of course, are free to do what you want with your camper.
As to the backflow preventer, there is the Venturi effect to consider. Air flowing along the camper wall will produce a low pressure at the opening of the inlet. This low pressure would suck out the black tank water if a vacuum break was not installed. This effect is seen more often at the gravity fill port on the fresh water side when campers depart with a full tank and yet arrive at their campground with half their water gone (sucked out of the vent line, also above the tank).
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