I use a Dremel multi-max tool to make accurate surgical cuts in very short time (Multi-Max
) and I use a solvent wash to prepare the old fiberglass, roving with epoxy or mat and Polyester resin, often over a backing of thin aluminium I attach with pop rivets. I use methods that are described on sailboat repair manual or in Resin/Epoxy vendors such as West Systems West System Epoxy - YouTube
finishing with my favorite product: Interlux watertite epoxy filler Interlux InterProtect Watertite Epoxy Filler
If you have a grinder, an oscillating tool, a pop rivet tool and a drill, you should be able to do the job yourself in a matter of a couple of days (waiting a few hours between each layer to harden). Note that the chemical reaction time is temperature dependent. The hotter, the faster it gets sandable.
Since you are not fixing a boat under the waterline, less expensive products can be used on a RV (such as good old polyester resin and mat fiber), but the steps are the same. Using Bondo on a door would be a bad choice because the bibration and stress is much too high to risk using automotive filler. Use Epoxy which makes a repair about six times tougher.