Honey wagon guy came and gravity drained the black tank and about 5-7 gallons came out. But when you look down the toilet, there's still standing water visible a few inches down from the floor. So there could be over 7 gallons in the tank.
He drained the grey tank and there was only a dribble in it.
I also got hold of a piece of heavy rubber garden hose and it went down into the tank easily about 5' with no resistance. Didn't help anything.
Water seems to be getting into the black tank somehow and the only way would be via the tank flush line. If that's the case, that's not good and a potential for contamination of the potable water.
Phoned the dealer and call not returned yet. When we're back home and the TT is parked, I can hook up fresh water and see if the black tank fills up. Still don't understand how we can see water below the toilet if the tank was drained. The only way to figure this out is going to be a trip to the dealer.