Re-routing furnace ducts in a old mobile home?

The stock setup has one joist channel the full length of the home that was tinned in to heat this place. I'm about to stick a video camera into the ducts, because I'm pretty sure there is either a break someplace or they have open ends. If I have a window open a crack when the furnace kicks on, the air rushes in like a hurricane!
I'm hoping to hire someone to put in insulated flexible duct and seal off the old one, but rodent intrusion is another issue.

If you want to spend the money and you find a good contractor, you could have them install a all metal duct system, and it could be insulated on the inside so rodents can't get in or get to the insulation. Later Paul

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.

I guess it'll be the foil and insulation ducts, although I'd like to wrap them with metal mesh to ensure no rodents (I can hear them fighting in the alley at night. They are too big for the cats to chase around here, but they are too fat and well fed to bother anyways)