Double wide mobile homes have a large crossover duct that transfers heated or cooled air from one side to the other. It is one of those items that may be installed by the factory, the dealer or the setup crew. Improperly done it can be the source of many problems and there is lots of room to shift blame. Any time you notice the house is not heating or cooling the same on both halves there is probably a problem with the crossover duct.
The crossover duct is another one of those mystery items hidden beneath the house where poor workmanship may take a long time to appear. The duct is not supposed to rest on the ground, it should be well insulated, and it should be connected at both ends with a metal or plastic band and sealed with metalized tape. If you have not checked on yours for a couple of years it would be good to look at it. Duct tape just doesn’t hold forever and you can lose a lot of heat or cool through a leaking, poorly insulated crossover duct.