i have a older mobile home with a miller furnace ran on propane in northern ny my question is can i add a cold air return to my furnace because it is getting outside air from under the mobile home

I belive your furnace is using outside air as the supply source that is heated and delivered to the mobile home for its heat. So in order to make it eaiser to heat this air you want to return the air that is already warmed up from the home back to the furnace where it would be reheated and then sent back to the home.

You cannot do this with the furnace system you have in place. These furnaces as well as the design of the heating system need the outside cooler air to properly work. If you re-cycle the air supply you risk excessive corrosion issues on the heat exchanger, perhaps ill effects of combustion, and even a fire.

They do make free standing heating and cooling units that can be set ouside of the home and then piped into the house.

I would not risk changing the design of the system without knowing more about the current heating system and design that is in place now. None of which can be done without looking at it.