The only tank that every indicates empty is the water tank. All others have "stuff" on the sensors. I usually add water to the tank - 5 gallons or so and when I get to our destination (if full hooks are available) I dump the tanks. Flush black tank with the flush connection.
I am going to try adding some "stuff" to the grey and black tanks next time to try to clean them a little. Not sure I would add bleach - very corrosive.
Found this on the net:
Clean tanks - USE WATER SOFTENER, DETERGENT, and CHLORINE BLEACH
This stuff is amazing and it works. Buy a couple of boxes of powdered water softener at the grocery store. You'll find it located with or near the laundry detergent products. I prefer Calgon Water Softener because it dissolves quickly in water. Cheaper water softeners work just as well but dissolve more slowly. Dissolve two (2) cups of the water softener in a gallon of hot water. Then, pour the solution down the drain into the empty tank. Use two cups of softener for each waste water tank in your RV. The tank's drain valve should be closed otherwise the softened water will just drain out. Then use the tank(s) normally until it is full and drain it normally. Add a cup of laundry detergent to the black (commode) water tank at the same time you add water softener. This will help clean the tank. The gray water tanks should already contain soap through normal use. Water softener makes the solid waste let go from the sides of the tanks. If you've ever taken a shower in softened water you know that after rinsing the soap from your body your skin will feel slick. That's because all the soap rinses away with soft water. Softened water also prevents soap scum from sticking in the tub. Get the connection? With softened water gunk washes away instead of sticking. The same thing applies to your RV's wastewater tanks.
I use a clear plastic elbow connector to attach my sewer drain line to the waste water outlet on my RV. It allows me to see how well things are progressing during a waste water dump. Before I began using water softener regularly the black water tank's water was brown, the galley tank's water was brownish, and the bathroom tank's water was white. The first time I added water softener to the tanks the water coming from the black water tank was actually black (not brown) and the kitchen tank's water was also black (not brownish). The bathroom tank's water remained white. That told me that the water softener had actually done what I had intended for it to do and made solid waste, which had been stuck to the interior of the tanks, let go and drain away. I added water softener (and laundry detergent to the black tank) to all the wastewater tanks for the next few dumps to be certain all the solid waste possible had been cleaned away. The waste water only appeared black on the initial treatment. I now add water softener and detergent to each tank once after every few dumps to maintain the system.
Too little water softener may not be of sufficient concentration to work effectively. Too much water softener will NOT hurt the tanks. So, if the amount you used didn't quite do the job, then use more the next time. Don't forget the laundry detergent.
Occasionally, I pour a half gallon of liquid bleach into each tank to deodorize, sanitize and disinfect them. I add the bleach when the tank is about half full, and then continue to use the tank normally until it is full and ready to dump. I no longer use the blue toilet chemical because it isn't necessary. I have no odors coming from my black water tank. The chlorine bleach kills the bacteria, which is primarily responsible for waste water tank odor. Generic brand liquid bleach is cheap and very effective.