Ever since we got our coach we have been strugling with an unpleasant smell from the black and grey tanks.
We bought a new ˝ million dollar (may have been slightly less
on sale) pressure valve (located under the kithen sink in our coach)and replaced the factory installed valve. Didn't make any difference. Re-installed the $5 original valve again.
We had Monaco check for leaks. No result / nothing wrong.
Then we went and bought a "Cyclone sewer vent" from Camping World. See this link.
Since this was installed we have had no bad smells whatsoever. It took longer to get on the roof than it took to install the new vent.
This is an easy fix for under $20 and we thought that we would share this experience since we most likely are not the only ones with this problem.
If you suffer the smell that we did - go buy this vent. For $20 you can't go wrong.