Yes you are correct that there is no way to actually clean up everything in the tanks. And it looks like you've done your due diligence to eliminate odors. Here's what I would do in your situation.
First thing is to identify which tank is the offending one. Pour a gallon of Pinesol down the drain and run the faucet to wash it all down into the gray tank. Open up the coach and vent any odors out. Now close up the coach and wait. Do you start to smell Pinesol or do you get black tank odors?
Other thought. Are you making sure to not have any power vents running when you flush the toilet? If you have a power vent running it will suck the smell up out of the tank when you're flushing.
Is there a washer dryer hookup in the MH? If so that drain trap might be dried up and is allowing odors out.