You should make sure the A/C company has matched the indoor blower with the size of the A/C.
Have a reputable company check the system out.

One problem with your type of application is the size of the supply and return ducts. You probably have a small sized supply duct down the center of the home. This should use a high velocity system, because of what they try to do. If you increase the air flow much more than what a 2-ton system needs, the air flow will be restricted too much to work. If you slow down the fan, you may get better cooling. The solution to the problem is to add additional duct work underneath the trailor. Use metal duct, to keep rats and animals out. Insulate the duct. It will need to be removed if you move the trailor, but it will allow the system to work properly.

I would make sure that the evaporator is the same size as the outdoor unit. Being that the manufactured home is only 1200 sq.ft., i would tend to think it might be smaller than 4 tons. I am assuming since you said the evaporator was clean & new looking that you should also be able to see the data plate on it. Somewhere in the model # you should see a 48, ex: MDD48DSD is a standard 4 ton Man. Housing coil. If the nunber is smaller, say a 42 (3.5 ton) or even a 36 (3 ton), the you need to get a coil that is matched to the outdoor unit.

If the home is pretty new (with in 10 years) the blower assembly will have the ability to deliver a full 4 tons of air. Thats usually how they come from the factory. Makes it easier on the installers. So... if the trailer is 1600 sq.ft., the it gets a 4 ton condenser & coil and hi spd. on the indoor blower. If its a 1000 sq.ft. home, it get a 2.5 ton condenser & coil and blower get set at medium low.

Definitely check to make sure the coil and condenser are the same size.

Hope this helps