I mentioned System Three because I have had good luck with their product, like the prices, seem to always have what I need, and send a very complete "how to" book with each order. They also have a technical support line. Try Welcome to System Three.com - Epoxy Resin Products
or 1-800-333-5514 and ask for a catalog. I'm sure that other brands of resin would work as well.
I know that epoxy and glass cloth will work on a fiberglass tub, but I'm not sure of the construction of your particular tub. Please double check with someone who knows your tub to be sure that an epoxy repair is compatable. There are a few materials that epoxy will not stick to.
As for tub removal I would bet that your '73 is designed much different than my '61. Once again, check with someone who knows your trailer. Hopefully someone here on the forums can walk you through that process. I think that my tub probably could not have been removed without first removing the bathroom counter and sink.
Good luck with your project!