Senseless sensors

I've been able to keep all the tank sensors in my TT working great except for 20 gallon unit for my kitchen sink (this is besides the 40 gal grey tank). It always reads 3/4 or full even though we use lots of soap and are carefull about what we put down there. Last week I mixed up put a whole bottle of Oxy-Clean powder in 10 gallons of hot water and poured it down the dang drain, then I emptied the water heater twice in it before leaving it there for 5 days. Then I drove 120 miles before draining it. No change.
Anybody have a home remedy for this?

Drained, rinsed, drained, rinsed, drained, the tanks were empty.
Put unit into storage last fall. Took the cat out of storage this
spring. Blackwater tank read full. I've given up on it, it
worked for about a week after we got the unit. Now it's almost
always full or 2/3rds full. I just know that every X number of
days I need to dump the tanks. We actually filled up the galley
tank to the point where water was standing in the sink, nothing
leaked. Never hit that point on the other two, thankfully.


About the only words I can add is to add some of those drop in
packages. Make sure the carton says they'll disolve solids.
I guess it's a chunk of TP drapped across the sensor. I tried
water/ice, etcetera and it did nothing.

Thanks Bill, the drop-ins sound like a good place to start so I'll try some.
I've been pretty lucky with the black tank. I think the tank flusher points at the sensors and keeps them clean. I fear that overfilling the sinks tank will have it gushing out the flimsy little vent under the sink.
I may give up and tear the bottom off the trailer to gain access to the tank and put a flusher in it too. I really don't want to do that though and I'm not even sure it would help.