I am new to this, but have almost completed my first season. I have not had any bad odors from the grey or black tanks.
This is my method that I am trying to refine....
1. when I get to the campground, I first dump the 1/3 full tank of treated water from the previous trip.
2. I then backfill the black tank to about 1/3 of fresh water from the campground and add a freshener additive/cleaner prepare it for use on the trip.
3. If the black tanks gets full during the trip, I dump it and fill it back to 1/3 for additional use.
4. When leaving, I make sure that the black tank is full then dump. I back flush a few times until the water looks pretty clear and free of solids.
5. After back flushing, I fill the tank with a little water and add a blue black tank cleaner and deodorizer pack. This treated blue water stays in the tank and sloshes around until the next trip.
My method takes a little while when dumping, but I'm happy because I get no odors. I do an abbreviated version of this for my grey tank.
I like this attachment for all of this.
Amazon.com: Camco 39062 Dual Flush Pro Holding Tank Rinser with Gate Valve: Automotive