Many of us actually use the soapy water from the gray tank to help rinsing out the black tank. Those with macerators can do this simply by shutting off the macerator and opening both gates. For us without macerators, I had to install a third gate to be able to shut off the drain and still open both tank gates.
I usually will drain the black tank and then shut off the third gate and open the gray gate. The soapy water will flow back into the black tank and when the two have equalized, I shut off the gray gate and open the third gate to drain that water out of the black tank. I'll usually do this a couple or three times (depending upon how much water was in the gray tank), until the water coming out of the black tank is clear. During all of this, I will keep the black tank rinse running.
Gas in the tanks??? Who cares? Both tanks are vented out the top of the roof.
Ron
