I had a very similar experience when we ordered our 31' Flagstaff TT. Had an '08 Tahoe Z71. Dealer said it would tow "no problem". While waiting many weeks for delivery I did alot of research on this forum & others. Some said I would be fine, others said no. I did not like the thought of white knuckle driving. Ended up making a killer deal on a new '10 Silverado 2500HD LB gasser, traded the Tahoe and kept the payments the same.
Towed the new Flagstaff like a dream but the gas mi. killed me. Then a few years later got the bug for a 5er...doing more research found the 2500 would do it but not as well as a diesel (for the size 5er we wanted) So...what I did and maybe you may want to concider it too...I "traded backards" found a '05 GMC Sierra 3500 CC DRW D/A with 89k miles in mint cond. Did a basic even trade, knocked $20 mo. off & shortened the term by 6 mos.
Got the new 5er and wow it tows it like a dream. Went from 9 mpg. with the gasser (7500 lb TT) to 12.5 mpg in the diesel.(10k lb 5er)
think about it
2005 GMC Sierra 3500 SLT DRW 6.6 D/A
2013 Crusader 335BHS TE