[JRTJH]

Well, "way back when we towed with Travelalls" (if you don't remember them, you're probably not going to ever collect on Social Security), we didn't have "black tank flush systems".... So, the usual way to flush the black tank was to go inside, run a hose down the toilet and fill the black tank over and over until it was cleaner (not as dark brown as the first dump)....

If you were without a hose or at a location that didn't have a water faucet, you couldn't "refill the black tank through the toilet"....

We had a way "way back then" to flush the black tank using the gray tank (remember, most trailers didn't have gray tanks, as we ran that water onto the ground). On those that did have a gray tank, we would close the terminal valve slide (the one that twists on that's sold by Valterra) and then open the black tank and gray tank valves. That would "equalize" the two tanks by flowing gray water into the black tank. After the tanks were equalized and before the black contents started migrating to the gray tank, close both valves, then open the terminal valve and dump the black tank again. There will be "about half a gray tank of gray water" left to flush the sewer hose after the black tank is empty. Just close the black valve, open the gray valve and flush the sewer hose as usual.

Yes, fortunately we're pretty much "beyond using gray tank contents to flush the black tank" so most people either have never heard of what we used to do or have forgotten that procedure.... Either way, yes you can use the gray tank to flush the black tank, but there are many, many ways that are more effective than equalizing the gray and black tanks with a terminal slide valve to keep the contents in the tank lines......