If you have a single discharge point. You can put a twist on valve on the discharge. Close the discharge and open both grey and black tanks. Allow grey water flow into black tank. This trick will give you access to the additional capacity left in the black tank.
19'Duramax w/hips,12'Open Range,Titan Disc Brake
BD3,RV safepower,22" Blackstone
Ox Bedsaver,RV760 w/BC20,Glow Steps, Enduraplas25,Pedego
BakFlip,RVLock,5500 Onan LP,Prog.50A surge,Hughes autoformer
Porta Bote 8.0 Nissan,Sailun S637
Correct Trax,Splendide