Lots of threads on this topic.
Levelness of the tank effects tank capacity and readings.
DO NOT attempt to cram more water into the tank.
The only vent is a small vent hose and the gravity fill port is much larger. Overfilling under pressure can bulge the tank in the middle causing it to "shorten" and slip off the steel ledge it sits on; dropping the tank on the ground.
It can also crack the tank; both very bad outcomes.
Lou, Laura, & Freya the wonder dog
2008 GMC Sierra 3000HD Crewcab SB Allison Duramax
2019 Flagstaff 8529FLS - Pullrite 3300
HAM CALLSIGN - KC3FFW