Why does air only blow out the vents on only one side of my doublwide when I use my heat or air

Standard problem. The jumper duct under the trailer has come loose or was not hooked up. It is a huge flex duct they use to jumper one side to the other sides duct work. That is why the one side only does not work. You are also probably heating under the trailer also and that will cost you $$$. Get it fixed.