I just don't worry about it and never check the gauge for the black tank. We full time and when the toilet "burps" when we flush I close the gray tank so I will have water to clean the hose when I dump, usually the next day. Been doing it for nearly four years now.
We do dump half a box of 20 Mule Team Borax into each tank and fill each tank around half full to keep them "sweet" when we move a couple hundred miles .
2011 Georgetown 378TS in FireMist
2013 Jeep Wrangler Toad
Kirby the Old Dog, passed but still in our hearts
Max E. Dog, the new pup
Eureka the Old Cat, still hangin' on