I'm in your situation somewhat. See my set-up in the signature. Loaded up, with the golf cart in the back, we're 26,500 lbs.. I have a RAWR on the truck of 7000 lbs. and depending on how much fuel is in the truck (I have an auxiliary tank) and how the camper is loaded, I'm 6800 lbs. to 7200 lbs. on the rear axle. Having the golf cart in the back plus a full tank of generator fuel helps.
A fellow member asked me about my experiences because he was contemplating a new F350 SRW or F450. He has an XLR similar to our Vengeance. This is what I told him.
Any where from 65 mph and lower, we are solid as a rock. Cruise control set and its solid and smooth. 65-70 mph and I can tell some difference. Not a worry and we usually tow around 68 mph on the interstates.
Above 70 mph and I need to stay extra aware. Its still solid but I can tell if some bonehead does some weird maneuver in front of me and I have to do some evasive action, there might be some trouble.
I had an F450 before this SRW F350. If the truck was primarily meant to be a camper hauler or I like to tow above 70 mph, I'd say get the dually. But being a daily driver, I'm totally happy with the SRW.