On the technical question of can you take molds from your trailer: yes, but that will be maybe only half the work - there are loads of little details that would need to be sorted out, like modifying one of the molds where the joint between the halves occurs.
You would need to take your trailer off its chassis and completely strip it of all its outside fittings - windows
, door, lights
, pop-top, etc, etc - so that the holes can be temporarily filled in and edges extended (the fiberglass when being molded needs to extend beyond all the edges and then get cut off). It would probably be easier to scrap your trailer shell after the molds have been taken from it, and then have the first new trailer shell for yourself, rather than to try to restore your current shell.
In practice, people take molds off other peoples' fiberglass products - it ain't right, but it happens. If the "other people" haven't been in business for 20-30 years, then it really isn't an issue. It's a bit like picking up something that's been lying on the side of the road for a year - technically it's still theft, but who would object?