When we had our TT, the sensors seldom worked correctly. I found some stuff that you poured down the drain and it said it would clean the sensors. It worked but was pricey and I decided that when the kitchen sink was slow to empty, it was time to dump the gray. That worked for me until we sold it.
We used Walex Elemonate Gray Water deodorizer and cleaner
I've heard from others that adding white vinegar into the grey tank will clean off the slime. You are supposed to add it when the tank is fairly full and let it sit overnight before dumping. I haven't done it myself, but it would only cost about a buck or two to find out.
2015 FR3 30DS
Towing 2016 Chevy Sonic LT