With all due respect, we've pointed out the that manual is obviously incorrect on some points as I've explained, so repeating that information is not helpful to the OP.
Further, my 2500 is a 2011 model with the higher tow capacities (not sure the vintage of yours). The limit for this truck is 12,000 lbs for a bumper tow, and is 20,000+ for a fifth wheel type trailer.
These trucks come with 2 manuals if you have the diesel and towing is referenced in several sections in each. You often have to piece together several tables and references to figure out what is your limits. That includes information not in the manual, but etched on your truck.
For example, when I was looking for a 5th wheel trailer, my upper limit is around 20,000+lbs. But in another reference, they state a maximum pin weight of 2500 lbs. On the door plate, there is a max stated of 2350 lbs payload. These last two weights, by definition, preclude towing a 20,000lb fifth wheel, as that weight could be 4,000lbs (20%). So I ended up buying a 11,000lb trailer (2200lb pin).
The same is true for a bumper pull trailer. On the hitch itself will be etched max tongue weight of 1000, 1500
lbs with weight distributing hitch. This will trump any table in your manual. By definition, an 18000 lb travel trailer could have a tongue weight of up to 2700lbs - far exceeding the mechanical limits of your hitch setup, as well as the max payload your truck is design to carry, WD or not. So an 18,000lb trailer is IMPOSSIBLE, no matter what the manual says.
So what I'm saying is you can't hang your hat on one table in one manual. Especially if it's fairly obvious that it is incorrect. All the manufactures have errors and inaccuracies in their manuals - it's a complex subject they are trying to explain and they really haven't figured out a good way to explain it to their customers.
Hope this clarifies a bit.