Thank you all. I have checked all the more obvious for obstructions and I guess I was hoping for something I'd overlooked.
I think I'll try to "poke" up into the tank and see if that's the issue. I'm pretty careful about allowing anything going down the drain knowing in time it would build up but of course rinsing greasy stuff from plates lets the grease go down and I'm sure after years it does build up. I just thought it strange it would drain only down to the 3/4 level after all of this time.
Glad it's not the black tank!