I have a 2005 Mandalay 40B and have owned since 20,000 miles, has 34,000 now. It has always had a bit of a sewer smell when driving. I do not run with vents open, both tanks have been bleached but o...
Most of the time the bathroom sink is plumbed into the black water tank, I was told this was to keep enough water in that tank. I find that if you close the drain stop on that sink there is no smell coming from that area while traveling.