Gary,
For what it's worth . . .
I've read about dumping ice cubes in the tank through the toilet and taking a short drive. It might work but I've never tried it.
On my first motorhome (purchased used), the dealer's tech spent a couple of hours with a flushing wand stuck down the toilet to hopefully spray off any deposits on the sensors. Of course, this usually only works when the sensors in the tank are fairly close to the toilet opening.
On my '05 Bus, I was able to gain access to the passenger side of the holding tanks. In the side, toward the top of each tank, I used a 2" hole saw to cut a hole in both tanks. This hole is so I can use a hose nozzle to hit the opposite side of the tank and clean off the sensors and rinse out the tank. When I'm done, I plug the hole with a compressible "test plug" similar to this -
Test Plug. I got mine at Home Depot.
Whenever I want to clean the tank or sensors, I just have to pull the plugs and "hose away". It works great for me.
Chuck