While I know very little about Airstreams, I have towed a little... I have pulled with both Gas and diesel trucks and I have to admit that when it comes to hard slugging, the Gas trucks just can not compete with the torque of a diesel.
One thing I will throw out there is this... I pulled the same trailer (A 16.5 foot Fun Finder about 3000 lbs loaded) with 2 identical Chevy 1500's Both 5.3, 4x4 4 speed autos, One had 3.73 gears the other had 4.10's. The 4.10 truck pulled the trailer MUCH easier. I was amazed! I would never have guessed there to be that much difference.
If you do end up pulling a larger trailer and find yourself struggling with it, you might consider changing the gear ratio as opposed to the expense of a different truck