If you ever are subjected to the stinky coach syndrome, it is usually a failed air admittance valve (also called vent). In our 09 both are located under the bathroom sink. Home Depot has the same ones for $11 and the superior Studor ones for about $30. In one rig I bought the cheap one but it didn't fix the smell. The Studor did the trick.
If these are good and there's nothing plugged up you should never have to add anything to either tank to prevent odors.
jor
You found something few LD owners know about.
The bath sink drain does not have a roof vent pipe, the one- way valve allows to sink to drain without bubbling. I have never seen one go bad.
Larry
I have never seen one go bad.
Count yourself lucky! ;D I had to replace them on two of my rigs when I got the black tank smell in one and gray in another. Actually, only one was bad each time but I replaced them all. No more smell!
jor