On our last TT that didn't have the black tank flush we would pull it half full of clean water and that did the trick for cleaning off the sensors. Flush it a few times to get it as clean as possible then fill it about 1/2 with clean water and pull it from home to the campground and then dump it. That usually fixed it for a while. Had zero problems with the current unit because the flush works to clean the tank.
Also; If I saw black water in a sink, any sink I would head for the dealer and get that fixed.
No man has a good enough memory to be a successful liar. Abraham Lincoln
2016 36FBTS Pinnacle
2016 F350, 6.7, 4x4, DRW, long bed
B & W Companion 5th wheel Hitch