Originally Posted by rockfordroo
It's actually a very handy valve. When I went to fill my fresh water tank to sanitize it this week, I realized that if I attached my fresh water hose to the a/f connection instead of the city water connection, and opened the a/f valve, I could back fill the fresh water tank from the pump connection. I didn't have to stand there holding the hose in the normal freshwater fill hole. When the tank got full, I closed the valve, ran the pump to pump the bleach water through the system, then drained the tank with the drain valve. Then I just opened the a/f valve again and refilled the fresh water tank. Did this several times to flush the bleach solution out.
You need to be careful with this approach. You can rupture your water tank or break its mounting brackets. The PrimeTime Sanibel RVs use this approach instead of a gravity fill. We have been instructed to fill only until the tank level lights indicate full and NOT to fill until water comes out the vent. See here for full details: Sanibel 3500 fresh water tank