Black tank smell.

Be sure to not have the roof vent going when you flush... I'm sure you knew that, but other than that you can try switching tank treatment chems and check the flap valve as described above.

wolfe10 wrote:
Yup, locate the source of the odor and fix it-- may be a check valve with bad flapper, may be a dry P trap, may be a problem at a tank or vent to tank.

Trying to make gray or black tanks smell acceptable is a good way to throw money away.

Yep. Aside from the small amount of odor that occurs when you flush, there should never be any odors from the tanks inside the rig. I would start with the under the sink vents with check valve as they are notorious for causing that type of problem. Just follow your nose and it should lead you to your problem.

The vent under your sink will look something like This.

Yup, locate the source of the odor and fix it-- may be a check valve with bad flapper, may be a dry P trap, may be a problem at a tank or vent to tank.

Trying to make gray or black tanks smell acceptable is a good way to throw money away.

Hi There is a pipe under your bathroom sink that has a flapper valve on the top which screws off. If the flapper is missing you will get that ordor.