2012 GMC 2500HD towing disappointment

[MJN]

I'm pretty disappointed with how my truck tows- Its a 2012 GMC Denali HD crew cab short bed (2500HD) 6.0L, 6speed auto, 3.73's, 275/70R18 tires. Supposed to be 360hp @ 5400rpm and 380 ft lbs @ 4200 rpm. I pulled our camper (34ft long) and it handled the weight just fine but would not hold 65mph. Dry/empty weight is 6615lbs, it was probably close to 7000 with some of our stuff in it. It was not windy but in tow/haul mode it would stay in 5th gear but then kick down to 4th, I would get up to 65 and it would shift back to 5th and slowly lose speed, then it would downshift again. I didn't want to sit and put my foot down and let the engine spin at 3000+ rpm for an hour. I know its a heavy trailer and 3.73's aren't great for such a big trailer either but I expected it to do better and not struggle so much. We didn't have any issues pulling other than just lack of power.