I believe you are correct, it would be very expensive. I think the 2015 Airstream Interstate 3500 extended is built with the things you suggest. I think it is priced at about 150K for a Class B, but is self sustained, solar and all. Tanks are small so you should have to drink from a stream and whatever other body functions are needed. I had an Interstate 3500 for a couple of years but like the size of the Axis much better.
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