There may be some residual adhesive left over if the decals have been on for a while. The decals peel off effortlessly when they're new. The glue is easily removed with some paper towels dampened with Mineral Spirits. If the trailer has not been left out in the sun a lot, you may find that there is no "ghost shadow" of where the decals have been when removed. If you have mild "ghosting" you can probably blend it in with a light
rubbing compound and fiberglass polish.