I think you made the right decision to keep the 2500. You'll never be short on power and the 2500 will handle the sway much better than a light duty truck without upgrades. In fact, I never used (or needed) a sway control when I had a TT (although admittedly it was about half the weight of your Flagstaff).
2015 Rockwood Signature UltraLite 8282WS Platinum, GY Marathon LRD, TST 507RV TPMS
2005 GMC 2500HD CCSB D/A, Curt E16, Prodigy P2, Garmin RV760LMT w/BC-20 b/u cam