Your ? about the airconditoning unit of 15000btus is a 1.5 ton unit. In a normal residential unit ,rating with average insulation is :
Normal aircond. one ton (10000 btus) =500 sqft
heatpump one ton (10000 btus) = 400 sqft
For children or intertainment add one additional ton.
sqft home ,memium would be 3 ton(30000 btus).
The Airstream is metel, and has about one inch of insulation inwalls and ceiling ,which is the equivelent of about r-4 rating.
The factory puts the memium in house and RVs and travel trailers. For RVs and travel trailers, overhead support and weight is very much considered.
the above information can be obtained from your local electric service company. Pamplets for residental may be obtained.
( An average size automable has a 4 ton unit , and it is metel)