I did a search using sewer odors and this interesting link came up
http://www.campingworld.com/forums/index.cfm/fuseaction/thread/tid/16281731/gotomsg/16285405.cfm#16285405
In this post, mention is made of downward air pressure during motion as follows:
"With the Fleetwoods, the Black water vent on top of the RV is designed to prevent water from pouring in when it rains. The Air Intake or Exhaust Cap on top of that Vent Pipe, Faces Downward, as usually next to the outside wall of the RV. This causes the Upward Wind Travel to pressurize that area and push air down the vent cap when traveling... or even during a slight wind traveling from that direction..."
This could be what some of us are experiencing. What do you think?