Assume that you have a 2 1/2 ton air handler that moves 1000 cfm of air.
Now assume that you are loosing 20% (200 cfm) of this air under the house thru duct leakage. 1000 cfm is being sucked into return but only 800 goes back in, so the house is in a negative pressure situation. Open the window, and the negative pressure sucks the air in.
Trailer ductwork systems are designes for high static and have a small trunk and all supplies come directly out of the trunk. Homes, however, do it right and have a lower static (and larger ) trunk and the supply lines run to the outer walls where the air is thrown up to create a thermal barrier that keeps the outdoor temperature from getting into the house.
Your trailer trunk is thin metal with a rectangular collar that comes out of the trunk and thru the floor
1) if the collar comes looks, air will come out and go between the trunk and
the trailer bottom
2) if the trunk sections come apart, air gous under the trailer
If you are staying in this trailer for a long time and dont plan to move it, it would be worth it to do it right and install a standard trunk (8" by 24") with 3" foil backed insulation and run new supplies to the outer walls.