I'm skeptical that heat will "shrink" the Ensolite, or cause it to react in any way. Ensolite is in fact so heat-and-flame resistant that it's used in some protective clothing applications ( link
That's also one of the reasons it's a preferred material for insulating boats ( link
), and perhaps part of the reason it was chosen for this application by those trailer mfr's who use/used it.
It may be that the glue beneath the ripples has given way, though...if so, I think a heat gun may only make that worse. Regluing, though tedious, may be an option.