Hello ZenFook -- Welcome to AIR Forums!
Patrick? The LS package was made standard for these Airstreams a year or two back right? And that now includes a spare tire as standard?
ZenFook -- I bought my Airstream when the LS package wasn't standard. So the LS features are not included in my baseline weights. And wouldn't you know it that the 720# tongue weight is unchanged in Airstream literature after LS was made standard. Here goes anyway -- added weight distribution gear (including hitch bar for the receiver -- 35+ pounds all by itself for my Reese Dual Cam), propane in the tanks & spare tire all cause me to figure I have a neighborhood 900# tongue weight with minimum personal gear in the trailer. This makes sense to be because the moment arm inputs of these A-frame mounted items puts very little of their weights back on the axles. I tow with a 3/4 ton, so don't think twice about filling my water tank before heading out. It would be good if I followed my own advice and checked my tongue weight with a bathroom scale
(same technique included in all Airstream manuals too). Tongue weight otherwise will be 11-13% of the gross trailer weight -- less can get into significant instability.
Your truck probably has a standard (not optional) Class III or IV receiver if you're towing that kind of weight, so wouldn't have to subtract payload pounds to do that upgrade. I used a 1/2 ton 5.6L Nissan Titan for my first few months towing the 25' FB SE. It worked very well. The Titan is unusual in having a 5-speed tranny -- the rear end ratio doesn't translate directly, but with the tranny combination it was supposed to be equivalent to a 4.10. It towed well but the truck's payload capacity caused me to limit what I could put in the truck.
I read frequent comments on the 5.4 liters. You could manage for midwest camping or until you'd be transitioning to a different tow vehicle. Mountains will be okay but take it easy -- mountains + interstate 4 lane grades is kinder. Diesel isn't necessary for these loads but 3/4 ton will be more durable for the long run. Let's see what 5.4L comments come along...