Holding Tank Filled Overnight

We have a 2003 Winnebago Minnie 24ft. The toilet drain is a straight down drop into the black tank. To check the level in the black tank, we shut off the water and any vent fans, open the toilet drain valve and use a flashlight to take a look - being very careful not to drop the flashlight. Much more accurate than the tank gauges. Our bathroom sink does drain into the black tank.

Since this is a "new to you" RV, I wonder if the previous owners left the black drain valve open, thus creating the dreaded "poop pyramid" which has partially filled the black tank.