1. I know my comment will open a can of worms.
2. I do not know why people washout blackwater tanks. Leave a few gallons of water in the tank and drive down the road and it will wash itself. If there is no place to dump after the drive home simply leave the small amount of **** in the tank. If it dries up over the winter (it probably will not) in the bottom of the tank no big deal. Again a few gallons of water in the tank on the first trip next year and it will be ready to dump when you get to the campground.
3. The sensors are not going to work most of the time anyway.
In summary, I would not do anything. I have been FT for 10 years. Before I FT I "summer camped" for 15 years. I never washout my tank.