Sealing heater from allowing outside air coming inside.

I think I understand what OP is saying, and that's also the way I interpreted his initial post as well.
It sounds like he is saying that the compartment that his heater is enclosed in is not sealed to the outside, allowing cold air (not combustion air or air going through the plenum) to enter into the RV.

I don't have any recommendations for a fix as I am not knowledgeable in such things. But that is how I understand his problem to be.