Check your furnace installation. It should take isolated fresh air into the plenum from the vent for combustion and exhaust it back out. How do you get air into your trailer through the furnace vent unless the plenum is leaking due to damage or improper installation?
If all is well with the furnace, check your fridge
to see if it is sealed on the sides from allowing air into the cabinet. This is much more likely.
You can insulate it using foil covered Tuff-R and aluminum tape...