The vans fiberglass parts leave a lot to be desired. That is putting it nicely. Most of the fiberglass parts from Vans....well, to maintain positive critique, lets just say it is easier to mask off, spray expando-foam, shape, mask, and lay up new. But definitely love PhatRV's posts above. Follow his advice too! Doing fiberglass gets less daunting as time goes on. It really is forgiving, and with care you learn to reduce the post-shaping and rework stuff. More often than not it is easier to shape foam and mask and just lay up new. Then you can lay down one or two more layers of glass mats (more layers compared to the pesky vans ones), and what you have in the end is more temp resistant and dimensionally stable...meaning it will hold its shape in the effects of the hot sun better. Again though I love PhatRV's slicing modification technique!
ANother trick I did on a friends RV8, he had me install his canopy and skirt, and his Vans's skirt, was rediculously out of wack. Huge out of fitment as yours. Don't under estimate the power of gorilla tape and super glue! Seriously. On my friends: I layed down Gorilla Tape on the turtle deck skin. Basically like masked the area off with this. Then I used fast setting super glue....and literally glued the skirt down into place. I then epoxied 2 full fiberglass layers down over those entire aft areas of the skirt. Once cured for 2 days, yes, TWO DAYS!, popping the skirt of was done with a gently nudge of pop sickle sticks. Gorilla tape remove, the fit and gap was puurrrfect! This trick works well to fix fitment issues.