Everyone that enjoys the benefit of a holding tank, at least everyone I know, after a few years, has to replace the Joker valve. Some call it a duck bill valve, or non-return valve, but essentially, it's nothing more than a rubber flapper valve that sometimes fails and allows foul-smelling water to seep back into the head (toilet).
In some instances, this is the result of crud build-up on the valve leaflets, which do not allow the valve to close tightly. This, in combination with a clogged vent on the goose-neck "Y" valve, and nasties growing inside the tank itself, can really produce a foul odor in the bathroom.
Cleaning out the tank scum can be problematic, especially when the tank does not have an access plate on the top. Just flushing the tank doesn't get everything out by a long shot.
The solution to this age old problem is Liquid Rid-X, a product normally used for keep home septic tanks clean. Just four ounces added to my 30-gallon, stainless-steel holding tank completely eliminated the odor overnight. Those little microbes went right to work and gobbled the nasties up, turning the solids into liquids in no time at all.
The other thing that was discovered was when urine is mixed with salt water it tends to create a nasty crust that is the consistency of semi-hard concrete. After a while it can completely block the discharge lines from the tank, thus requiring replacement. Ordinary vinegar poured into the head and flushed into the tank eliminates the crust, dissolving it away in a matter of hours.
Hope this information is helpful to others on this wonderful forum,