Bill won me over with his safety argument.
Today I bought a new 2007 Ford F250 6.0L Diesel. It's got Lariat trim, TorqShift tranny, TowCommand, and a few other bells & whistles. I wasn't planning on buying new, but Ford is offering some nice incentives on the '07 model (since the '08 trucks are now on the lot). This worked out great for me, since I really wanted an 07 -- I like buying models that have had all of their "teething issues" sorted out, rather than one early in a production run.
I am happy with the deal, and especially pleased to have a "whole lotta truck" that should handle our Airstream with ease. Thanks to all for sharing your thoughts and experience. I am now officially ready to start Airstreaming!