If it is not the formaldehyde in the trailer walls, it could be your gray tank is dirty. The smell coming from a dirty gray tank can be really nasty.
Before you store your TT do you flush and clean out both of the holding tanks??
My Rv's never get parked without first flushing out both tanks with a couple of fill and dump load of fresh water. Then I put a bottle of lemon cleaner down the gray and a little bit of water. Same thing with the black after flushing I pour a little bit of black tank chemicals and then some water to cover bottom of both tanks.
I Never put them away without flushing and never put them away totally empty.