cephoto: I've yet to see any of those red/yellow/green light tank level indicators work with any degree of accuracy/reliability and I don't trust them at all. OK, I agree, they do work when the vehicle is/was new or after the tanks have been power washed. (for a while) I just flushed my black/gray water tanks today but the level indicators still read full. I KNOW they are empty.
I don't mean to be arrogant or rude, it's just after years of shop training/experience, ALWAYS CHECK THE MOST SIMPLE THINGS FIRST is etched in my DNA. I wish I had all the hours back that I've spent pulling my hair out trying to diagnose a complex problem when the answer was a simple blown fuse or a flipped circuit breaker.
Remote diagnosis is always a challenge.
Chum lee