16k BTU does not sound like enough to keep you warm in an old 28 footer with outside temps below 40 F. I installed a 30k Suburban furnace in my 1966
24' Trade Wind, however I elected to use under floor ducts, so I will loose heat in the "basement". Most Airstreams with under floor water tanks have heated tank cavities, using more heat. The NT 30 Suburban furnace seems like a very popular model. My trailer had a 22k BTU unit in it before the replacement.
watt electric heater does an admirable job on cool mornings, like 50 F.