All this is something that is very costly and labor intensive and toxic so do extensive research B 4 you make your decision.
The molds are reverse molds meaning that they have a mold where they shoot the clear coat on the inside then add the
fiberglass and then extract from the mold.
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For you to make a mold you would have to do the exact opposite by taking your camper apart then waxing up the outside of the camper then shooting it with clear coat then
fiberglass then building a ridged wood frame to the outside of the mold then more fiber glassing and when all is done, you extract the original camper part out of the mold. You now have a mold where you do everything over again this time making your new camper part.
Most of these campers are fiber glassed using a chop gun where the epoxy and
fiberglass strands are shot/mixed from the gun and the gun chops the fiberglass as it is shot and then you roll the fiberglass and epoxy to get saturation of the epoxy and the air bubbles out.
You might want to visit a boat builder or take a tour through a camper factory to get a first hand look at what it will take.
My experience: I had a friend in Germany that manufactured FiberFab sports cars using the molds and chop gun process and I was in his 2 person factory many times.