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...
Probably a silly question, but do you have the plugs pushed into the sinks and tub? What some people think is sewer smell can also come from the grey tank. Some of that stuff really stinks too.
You can also drive with the bathroom vent open just a crack.