I actually owned a 2007 Keystone Outback 21RS and towed it with a 2006 F150 Crew Cab 5.4, 3.73 etc. Pulling wise the 6.0 will be plenty strong especially if you can get one with the 6-speed. However as others have hinted at the hitch weight + payload of gear and passangers gets eaten up very fast. We put a lot of the weight in that trailer vs carrying it in our truck bed. But we still bounced around when we had a fully loaded truck.
For the minor price difference go for the 3/4-ton Suburban with the 6.0 6-speed.
