And that may be part of the reason they gave up on this project! It's amazing what can be done to bring old Ensolite back to life, and it's far less work than replacing it. However, the deed is done, and you want to move on...
I like what Scamp
uses: foil bubble insulation covered with marine hull liner (known here, mostly affectionately, as "rat fur").
The insulation is available through Scamp
or from big box home supply stores under the trade name Reflectix
Marine hull liner is a carpet-like material, but thin and stretchable to conform to the curves of a boat hull (or molded fiberglass trailer). It's moisture and mildew resistant, soft to the touch, and quite durable (Scamp has been using it since the early 80's, I think). You can buy it through Scamp
: Scamp Headliner
You can also order a similar product through a marine or automotive products supplier:
HullBlanket Headliner Hull Liner Carpet Type - Conforms to Shapes
72" Marine Hulliner (Headliner) Fabric
From what folks have posted, it sounds like Scamp's version is thinner and more stretchable, so it might make for an easier installation for a first-timer. You may want to do a Google site search for details from forum members who have done this project (blue bar at top of screen under log-in information, "Search," and scroll down to the Google search feature at the bottom).
A belated welcome to the forum, Chancy and Jody!
Best advice I have heard is not to wait until everything is "perfect" to start using the trailer. Make sure it is road-worthy (tires, bearings, coupler, lights) and clean, and start camping. Experience will help you prioritize the restoration.
Enjoy your new trailer! We like pictures...