Do you warm up camper before heading out for winter camping?

Photomike,

Sounds like the various air leaks or unsealed voids are being enhancing the sucking action of the fast passing ambient air flowing past the stove vent opening which then creates a venturi sucking action. The same way the old carburetors used to use the sucking venturi action. It will create a low pressure situation in the TC which forcefully sucks outside air in thru even the smallest of openings or not tightly sealed areas. Sounds like you really do have some sealing etc to do!