In the auto repair business, we used to remove large decals with a spray adhesive release agent. It is slow and difficult, but it can be done. The up side is that if it is done right you do not have to repaint.
If there is a better way to replace the 3-M Shield, Beaver Coach Sales service department will know about it. Their Body and Paint Shop works with them on a regular basis. If you are not going to go through Bend any time soon, you can call them, and get a good idea what is involved before you talk to your local RV paint shop, but if it is possible to go to Bend you will probably be better off.
Gerald