I have a 1976 Trillium
and discovered that the "show" fiberglass on the counter, seats, etc. was actually in pretty good shape and didn't need painting
once I'd applied the necessary elbow grease to get it really clean. So I'd say don't rush and remember it's not a new trailer in any case. I am going to fill a couple of bad scratches and may apply some Zep.
The inside of the cabinets and storage compartments were a different matter - stained to the point I didn't want to put our stuff in there! And the porta-potti garage? You don't want to know! And the inside of the cabinet that once contained the furnace
was just rough, hairy fiberglass, original build I think. So I cleaned and sanded all of these, applied some resin to any hairy fiberglass, and then three coats of acrylic porch and floor paint (recommended by Trillium Tom and my hardware store). My paint expert says don't prime anything you're going to walk or place heavy objects on. I had it tinted a nice Trillium beige and they look nice and clean and stands up well.