I did a full interior restore in my 77 Ventura
Make sure the floor fiberglass is in good shape. If not put down a new layer of glass first. Instead of trying to bond the new plywood floor to the wet fiberglass I did the glassing first and then glued down two layers of plywood on the new fiberglass. I put down 1 sheet of 5/8" and 1 sheet of 1/2". This made the door frame work well. Under the trailer I put wedge shaped treated lumber to re level the floor taking up the space between the steel frame and the fiberglass floor. You must put some sort of reinforcing here or take trailer off frame and install new steel framework. I used wood. As for any holes, put some glass mat down to seal them from water penetration.
I used premium adhesive to bond the plywood floor to the new fiberglass. And also glued the 2 layers of new plywood. This will make the floor solid as a rock.
Paul Lewis has some good ideas as well.
Good luck, its worth it. These are great little trailers.