Hello Andrew -- Welcome to the Forums!
The posts by wbrownrr really point to a big issue in towing. Karma!
Looking at Dodge's website
for 2006 models (your owners manual would speak best for your truck), under capacities/weights it says you could put 1440-1490 pounds payload burden on the '06 truck. This spec always is for the base model without options that add additional weight to the truck; the added option weight must be subtracted from the payload allowed. Vehicles in this category should come with a standard 2" hitch receiver so that won't hurt you at least. This issue probably rules out adding an extra gas tank.
My hitch bar and ball weigh close to 40 pounds. The weight distribution gear and propane in the tanks all add to the published tongue weight for my '06 25' Safari FB. My Safari's published empty tongue weight is 750# (720# for the SS). Options count there too! The valuable LS package adds a spare tire right behind the A-frame. All these add-on weights give a real-world tongue weight of 950-1000#.
If you subtract that tongue weight from your truck's payload you'll only have a few hundred pounds for passengers and gear in the box. WD gear does not reduce tongue weight
-- it just transfers some of it to the tow vehicle's front axle. I had a 1/2-ton truck a year ago when I got this Safari and didn't use a truck cap or carry much else in the box. Newer Airstreams of this length are about 1000# heavier than 30 years ago. I towed safely with the 1/2-ton but made plans to replace it by spring of '07. That replacement happened early ...
... but the message for you is that you can tow a 25' SS with this engine but preferably with a high rear differential ratio. After hitching up you'll have to be very aware of loads on the tow vehicle itself. The discussion can get much longer, but high tow capacity and GCWR do not allow an owner to safely exceed the tow vehicle's payload capacity. That should be in the manual too -- it sure is in my 3/4-ton GMC's!