I looked at an F-450 a couple of weeeks ago, the dealer sure tried to make a deal. If it had been available with the Cummings or Cat I may have got real serious about trading the ole F-250 for a better tow truck. I am running a 03 F-250 Crew Cab FX4 and tow a 36 foot all fiberglass Montana 5th wheel, she does ok in the towing department, but I would like the bigger truck to provide better braking. I worry every time we travel through the Rockies, and keep my eye peeled for the run-a-way truck lanes just in case.
You sure see a lot of F-550s out on the road with travel trailers behind them, I'd bet there will be a few of the 650s out there too. I guess you can justify the Ford, Freightliner or one of the other large tow vehicles if you travel full time.