Bruce is your current material fiberglass and is it molded to shape. Most car hood liners today are molded from fiberglass and trying to get a molded shape for your coach may be impossible unless Foretravel has them.
If it is a flexible fiberglass then you can try to find a distributor of air conditioning and heating products. You probably want to try and find Duct Liner insulation. The fiberglass duct liners have a coating and/or facing on one side to make them more abuse resistant.
You can also obtain adhesive used to attach the insulation to the inside of metal ducts. I would also try to use large washers and screws to further secure the insulation and to hold the insulation in place until the adhesive set.
If you can't find a distributor of the insulation, see if you can find a large sheet metal duct shop that does commercial duct work as they will be using the material and would probably sell you a roll as well as adhesive.
I believe the insulation comes in 1/2, 1, 1-1/2, and 2 inch thickness. You can check the websites of Owens Corning, Knauf, Certainteed, or Johns Manville for the duct liner products they make. Just Google the company name and add duct liner to that.
Pete - Full Timing
2000 Country Coach Magna 40' Indulgence, CAT C10, #5892
2019 Ford Ranger XLT