For the black water tank (toilet), look down the toilet while holding the flush mode open. It's a straight drop into the tank.
For the gray water (shower, sink), the shower drain is the low point opening in the system. When the water comes up through the drain, it's full.
There are smarter people on this site that probably have better ways of checking and I am sure that they will chime in with their ideas. Welcome the the Sunline lifestyle. You will find that it will be the best investment for having great family time.
2012 Rockwood Windjammer 3008W
2011 Ford FX4 Crewcab Ecoboost
Equalizer WD Hitch System
2001 Coleman Utah
2006 Sunline T-2499