I went from a 1999 F250 7.3 to a 2011 F250 6.7 in November of 2010 (both have long beds). Never had a truck camper on the 7.3 and it was two wheel drive. The 6.7 4 door 4wd Lariat with camper and towing packages. With my old 3K TC and towing a 16ft utility trailer with about 1500lb's on it (including a Polaris RZR) I average about 12.5 mpg driving most of the time at 60 to 65 MPH. With TC on and trailer hooked up the rear drops about 3 inches and the truck has no suspension mods. Camper package included a lower geared rear end, sway bar and more springs front and rear.
The 99 and 2011 both pulled my 32ft bumper pull TT, with the 6.7 I gained about 3 MPG and tend to tow faster because it pulls it so effortlessly.
Absolutely no regrets on the 2011 purchase and FYI the 1999 is still going strong and looking good for the guy that bought it.
I can't help you with warranty issues, my 6.7 has not needed any repair yet and the warranty expired last November.
