Airstreams are thermal sieves. Made of aluminum inside and out there is few less effective materials, from a thermal consideration, they could have been made from. Just as you are willing to accept that they can be heated by the sun during the day you have to accept that they will loose heat during a night dropping into the 30s.
With 2 people and 2 electric heaters, about 3,500 watts, I can only hold my trailer to about 10 degrees above the outside temperature. When it is 17 degrees outside we get up fast and get dressed. You have to look for a heat source other than human. Try it one night without the heater turned on.
WBCCI 12156 AIR 3144 WACHUNG TAC NJ6
2004 Excursion 4x4
1991 34 ft. Excella +220,000 miles, new laminated flooring, new upholstery, new 3200 lbs axles