1. Forget about the tank sensors, they are useless and never accurate.
2. Buy a clear plastic sewer connector, connect it to your trailer sewer outlet, then connect your sewer hose to the other end. Now when you empty the tank you can actually see when the tank has emptied as the the water will be clear when the tank is cleaned out.
3. You don't need a sewer back flush to dump the tank. It is nice to have one and most dump stations have them. You can always use about 3 one gallon containers and empty then through the toliet. Close the black tank handle before you dump the 3 gallons of water, then open the tank again. Repeat as often as necessary until the water coming out of your black tank is clear. :)