The decals are fairly easy to remove using a heat gun. Any residual adhesive can be removed using a variety of solvents, bug remover etc.

The problem arises with the areas under the decals, now differing in shading with the surrounding aluminum or paint. Even polishing the metal, will still leave an outline of the original decals. If OEM decals or new larger decals are applied to these areas, the shading can be hidden. An alternative is to hand mask the areas, and shoot a new trim scheme with paint. You can copy the newer styles, even from other trailer manufacturers, and easily modernize your trailer's exterior appearance.