On long visits with full hookups, I make sure to start with the black tank empty and then close off the valve. I leave the gray open until the last night and then close it. This will be my rinse the next day after I dump my black tank before leaving.
Doing it this way allows for long showers without tank fill issues yet ensures the black fills up enough to flush out solid waste when opened. Although some will not hookup to sewer until ready to dump tanks, this hybrid setup works best for me. I do not suggest leaving the black open the entire time because you will not get enough liquid through the tank and solids will build up.