We came back from a couple weeks in NV and NM, parked the rig in the back yard and all was good! A few days later I notice an odor in the trailer - it took two days to narrow it down to the bathroo...
After putting the water in the traps I still had an occasional odor, so I took Phil's advice and picked up a couple mechanical vents, changed out the old ones - just walked out to check for an odor after about a week and *bingo* - no odor! I think the valves may have been the culprit all along. I also think the grey water tanks smell worse than the black tank...
Case closed! Except for the occasional 'blue haze'! :B