MountainGirl, while I agree that a whole house trap may solve the odorous roof vent problem, please be aware that in many jurisdictions whole house traps are prohibited. The reason for the prohibition is that the trap increases the potential for sewer pipe clogging, and it is difficult, maybe impossible, to run a snake through the trap. In addition odors may still be produced in the individual traps on the various, seldom used fixtures in the house which are vented to the roof vent. Also, the cost of the roof vent filters is much less than the typical cost of installing a whole house trap.
I have no doubt that the installation of the main sewer pipe trap solved the odor problem for Retired Plbg., but I think that you should be aware of the alternative approach. Contact the local regulatory agency to find out if whole house traps are allowed, and then get a cost estimate for each alternative before you decide on a course of action.
John Aldrich (Septic Tank Yank) Septic System Consultant Timnath, Colorado USA