Might be a lot of truth here....
These are photos of my tanks during the manufacturing process.
It looks to me like the outlets are right at the bottom of the tanks so they should dump completely as long as the hitch pin is higher than the rear bumper.
The left tank (from the back) is the gray and the one on the right (with the white hose - black water flush line) is on the right. What confuses me is there is a probe at the very bottom on the left of the outlet and then 3 probes on the right of the oulet.
I assume the left probe is the ground and the 3 probes on the right are the lights. You can see that the lowest right probe is very low on the tank and the highest probe is also not at the very top (your observation seems right on!).
It should dump empty from my view even if the trailer is level. I will try another dump later today and use another heavy flush cycle.
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW