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.
Step 2: Add/glue Screen Into 2" Fittings
Step 3: Glue One of the Fittings to End of 2" Pipe
Step 4: Fill With Aactivated Carbon.
When I took this photo with my old cheap cell phone, you could see the carbon up to the top of the pipe.
Step 5: Install
Use the vinyl tube to make connections to tank vent and outside exit vent. You might want to use hose clamps.
The 1" vinly tubing (hose) is the size that fit the pipe fittings on my boat. Your sizes may be different.