Hello Michael -- Your profile says you have a 2006 F-150. Your manual would be the definitive source, but a similar perspective is at 2007 F-150 specs
(click on Payload Package Selector). Your truck has a maximum GVWR. You will find that emergency maneuvers and durability are enhanced if you don't exceed 85% of that GVWR. Curb weight (empty) + maximum payload = GVWR.
Figure out what aftermarket options you've added to the truck (truck cap?) and what you want to carry (driver + passengers, pets, cargo in box). Add any prospective Airstream's tongue weight
-- probably add 200# for weight distribution gear, LP in the tanks, and minimal
personal gear aboard. Actual weighing is the only way to be sure of the hitch weight range when a travel trailer is loaded camp-ready (see http://www.airforums.com/forums/f464/hitch-weight-bathroom-scale-24195.html
). You need to stay honest about being within this limit regardless of what public relations big tow capacity number comes with your package. As many of us have found, if you stay within a tow vehicle's payload capacity you'll probably rarely approach tow capacity or GCWR. Manufacturers compute these latter numbers with an un-optioned truck, almost no gas, and the lightest steeplechase rider they can put behind the wheel -- not very 'real world.'
Vintage Airstreams can be about 1000# lighter than their modern length equivalents. It would be dicey trying to tow a heavy 28'. You might need to load with some care to pull a newer 25' with a 1/2-ton truck -- just pay attention to the payload capacity and you'll be okay. I know you're putting a lot of consideration into your pursuit of the perfect Airstream for your family!
What engine do you have in your Ford?