Ok, more data for you guys...
I ran the experiment on the black tank (twice to make sure). Both confirmed the following:
Ran the hose straight to the toilet with flap open. At 25 gal, the black tank was filling up to the point that the water line was meeting the flap. Black tank sensor read E the whole time.
Next, I hooked the hose to the backflush port on the exterior of the trailer. Same results: 25-26 gals and the water line was to the flap, yet the black sensor read E.
So it seems I have an overreporting gray sensor and severely underreporting (or inoperable) black sensor. (However, the black sensor DID go to 1/3 when camping.)
Seems like the sensors are no good despite the trailer being brand new.