I'm a CPA so I can value anything
I'd start by looking at competitors, then add a premium since Airstreams command a premium price.
However, if the purpose of the rental is to see if you would like to own an RV, you don't need an Airstream to do that, and no premium is justified. If you were deciding if an Airstream was worth the premium over another trailer, you'd be wise to rent both. So the value of an Airstream rental to one user may be much different from another. As an example, I already own an Airstream, so there would be little value to me in renting something I already have the use of.
The premium price of a new Airstream is at least partially a function of the higher resale value and longevity of an aluminum tube compared to the relatively short life of fiberglass boxes. Since the renter receives no benefit from the resale value, the premium cannot be only a function of the higher purchase cost.
And that's the most that I'll ever expect to use my Masters Degree in Accounting here on Airforums!