I think the dealer and GM told you incorrect information according to the US warranty guide. The tires should be covered 100% for defects within the first 12,000 miles, then prorated after that.
The Denali has a lower tow rating due to the magnetic ride control suspension. The NHT would give the highest tow rating, but is not available on a Denali. The dealership probably just quoted you the highest tow rating for a Sierra 1500. Another thing you have to watch for is the payload, which is on the sticker inside your drivers door. Most fifth wheels will probably put you over your payload.