[prndl]

I have done that a few times. While camping without hookups the black tank does not get filled as fast as the galley and gray tank. With the extra valteravalve at the end of the line we just open the gray or galley valve and then the black tank valve and we backfill the black to gain some extra room. Close them all up and we're good to go for a few more days. Works when dumping the tanks like you have done also.
I had to add the fourth twist on valtera valve while on a trip in Canada and the OEM black tank valve would not close. When we got home I installed it permanently.