Generally values can be established by looking at the current sales values of similar aged models. Of course the problem in your case is that there aren't enough Amerigos changing hands to fairly guage a market value.
I would try watching eBay, and the 'for sale' ads here and on the Casita
Club site as well as some of the other advertising places and collect comps on similarly equipped mid-70s fiberglass trailers. Even though they won't be Amerigos, your insurance company will probably accept the value of similar brands of that era as comps for yours.
Some policies allow for a 'declared value', but you generally still have to support that value with some evidence of it's value if you make a claim.