There is no question that the diesels have their advantages, but the gas engines do pretty well. Pickuptruck.com did a comparison in their 2010 shootout. On the 16% hill climb pulling a 10,000 lb trailer the times to the top were:
25.6 Chevy diesel (32 mph @ top)
26.74 Ford diesel (@ 31.1 mph)
30.75 Ford gas (@29.5 mph)
30.95 Ram Diesel (@24.2 mph)
35.6 Chevy Gas (@24.9 mph)
36.4 Ram Gas (@22.1 mph)
So one gas beat a diesel up the hill and two gas engines were going faster then one of the diesels. Off course it is going to depend on axel ratio's etc. but modern gas engines can be pretty competent.
I have a 5,000 lb truck camper on my pickup and I'm pretty impressed with it. a big part of the reason for not going with the diesel was the nearly 1,000 lb hit I would have taken in payload capacity.