There is no prize for filling your tanks to the tippy top.
When I'm hooked up to sewer I leave the gray tank open. This will eliminates the possibility of tank overflow. We have a washing machine plus a dishwasher and I don't want to take any chances with a 55 gallon gray tank.
I empty my black tank anywhere from 1/2 to full (45 gallons). I then flush it two or three times, add a few gallons of water and close the valve.
Of course, if you're dry camping ...that's another matter.
Fulltimers! - Currently in Gainesville, FL - working as Webmaster & Special Projects Manager