I took an idea from Warren and it worked great. First I emptied and flushed the tanks. Then filled the tanks with water and 1/2 bottle, say 12 oz per tank of PineSol the blue kind and let it set a week or so in each tank.

You will not believe how it cleans the tank.

I also took all the drain vents, I call them island vents, out of the coach. I had four, one under the kitchen sink, under the two vanities, and behind the Headhunter toilet. All four were not working well and a 1-1/2" plastic pipe plug was installed at each location. No issues at all, no slow drains, gurgling or bubbles.

Best of all no smells.