The reason for the moisture on the walls is that the moisture in the trailer is condensing on the walls because they are colder then the rest of the surfaces of the home. Much like a ice tea glass with ice. It sweats. To prevent this occurrence, you need to control the humidity within the home. Be sure to run exhaust fans when cooking and showering. If you have no fan venting to outside, I would you spend the money on having them installed. In the interim I would suggest that you open a window to allow for the excessive moisture to escape out of the house.
Left unchecked you will begin to develop mold inside the walls as more moisture will develop inside of them.
Lastly I would also check carefully the ability of the furnace and hot water heater if they operate on propane, or natural gas that they are venting properly. When gas burns a tremendous amount of moisture is created as one of the byproducts of combustion as well as other harmful gases. So be sure that you have a CO alarm in the house just in case.
That takes care of the "water vapor" issue. Another idea would be to address the "cold" side in tandem. Most moblie homes are not well insulated, while not as easy to do as in a stick built home. It is possible to improve the wall insulation, Rigid and spray foam have higher "R" values which work well in the thinner wall cavities.
Thanks everyone for your Help!
I would cover the ground under the mobile home first, If you can get under there. Clear plastic would help a lot.
I had a moisture problem on an outside wall last year. The air vent was on the floor below the problem area. feeling the cold air on the wall, I grabbed a deflector ( the kind with the magnets to hold it in place) and sent all that cool air out into the room, rather than straight up the wall. Problem solved, the wall is no longer moist and I feel we are getting more of our money's worth from our central air.