The furnace that came in the Airstream should be a propane fired 25,000 b.t.u. unit. Unfortunately over time they became defective and leaked carbon monoxide and would be killers.
The unit you have is not something Airstream supplied . It was most likely installed by a previous owner.
Airstream today is using a 25,000 b.t.u. unit in the larger trailers this would be the Suburban NT 25 SP .
I ordered a Suburban NT30 SP it's a 30,000 b.t.u. from Vintage Trailer Supply to replace the original "Killer" furnace that was in our '69 Tradewind . It may be over kill for the southwest but you never know when your camping in the north in late fall.
Glen & Jane 1969 Airstream 25' Trade Wind
2014 Toyota Tundra
1998 Chevy Tahoe
2001 Casita 13' Patriot Deluxe
2004 Chevy SSR
+ 4 other trailers & 6 other Cars & Trucks