These holding tanks all have a vent, or they would explode, and if you thought odor was a problem....
Activated carbon, available as aquarium filter media, will remove the odor from vent gas.
I built a home made odor killer for the holding tank on our boat which solved the problem.
Step 1: Parts
2" PVC pipe, 24" long
2" fittings for each end, the glue-on type. These fittings need to reduce down to a size where you can slide a length of clear vinyl tube over it. So, I used fittings that reduced town to a short length of 3/4" PVC pipe so that I could slide 1" vynl hose over it tightly.
1" vinyl tube
Activated carbon that can be bought at a pet store
Fine screen, like window screen, of fine wire screen.