My gray tank sensors seem to read fine, but like others, the black is iffy. The past couple of "dumps," the thing still read 2/3, so I just waited until the "full" light started to flicker on/off while pressing the switch. No problems. I've been using a Calgon mix in the black tank, but the trailer has been stationary. This last dump, I took a chance and actually ran the tank flusher for several minutes (leakage horror stories had me scared of it). Sensor read 1/3 after it was over. And it didn't leak...yet.
As for the washer/dryer flood, it's a good practice to open the tank valve before starting a load. To clean the gray tank sensors, use the Calgon mix. There should be enough soap in the tank already to help, but you can add some powder laundry detergent to the Calgon mix and driving around will help "scrub" the inside of the tank. Same process for the black tank.
Back to the Calgon. I've been using it exclusively in the black tank and have not had an odor...at all. To use up my stock of the AquaChem, I put a bottle of it in the last time I drained it. Didn't take long for the wife to complain about the smell when flushing (actually the smell was the chemical, not the tank contents).