John,
When you dump the tanks, do you have the fan or air conditioner on?
I noticed this week when I dumped the tanks, that I forgot and left the gray tank valves open for a day. Whoa. With both ACs on, a lot of air was being sucked in and I was sucking fumes (I assume) from the CG's waste system via my sewer hose and tanks and maybe via the studor valves or even p-traps. Dunno. Shut the valves - no more stink. Same could likely happen with Fantastic fans going and tanks valves open.
Just a guess.
Jim