Sewer smell in RV

I started using the "geo" method of putting water softener + laundry detergent down the toilet when the water will slosh around (before and after a camp/dumping for example) and I have noticed the "cabbage" smell is ours has been much less offensive, supposedly you have to do this a few times before it totally works, so we are hoping the smell will be totally gone next time.

Under-sink valve: they are really cheap valves, use only a little piece of rubber that resembles a chunk of inner tube to keep fumes from rising into the trailer. Unfortunately, the flap is fighting gravity along with air currents.

I replaced mine with a higher quality unit that I got at Menards. But the same or similar valves are available from Amazon. Expect to spend about $20, not the cheap replacement that costs around $5. Or expect to get recurrences from time to time for no apparent reason.

Yep. I vote for the under sink valve. Mine was loose on the new trailer and went defective on the old. Found a new one at Home Depot under $20.

I had this problem in my new FW. The flapper valve under the sink was loose. I hand tightened it and the smell did not return.

If it is evaporation of the p traps, a little Wesson oil or other cooking oil in each drain and the toilet will greatly slow the evaporation process.

Some RV's don't have roof top vent pipes from each drain into the holding tank for grey water waste from sink, shower etc.

This explains what a "under sink flapper Valve" is and how to replace it.

Everything about RVing explains it!

Thanks I will give that a try, I figured it was something simple, thanks again.

A very common problem. Just go out to your RV when it sits and put a little water down the drain once a week. The traps will dry out and allow grey water smells in unless you do that.