I really like the fact that the beltline trim ends just before the windows, and that the window frames are bare aluminum instead of paint. I'm definitely making both those mods when I finally have ours painted!
Right now I have that "sour grapes" thing going about furnaces, and there are definitely people who'll pay extra for the original '70s upholstery in good shape. Remember that people tend to pay more for shorter Airstreams and Argosies in a given condition, so if there's nothing wrong with yours other than what you stated, hold out for a buyer to come close to the #s other people are tossing out.
I paid a little more than that for our Argosy that is much less cosmetically appealing (original paint), but it had new PEX plumbing, a new 12V
converter, all new seals on doors and windows, a newish water heater and all new screens. Paint is frankly a bigger undertaking than some of those other systems, and someone walking up to your shiny trailer should be favorably disposed to it from how nice it looks. I say ask north of $6k, hold out for well over $5k.