With the burb this summer we pulled approx 5500 lb camper to Destin (about 750 mi round trip) and got about 10-10.5 mpg, drove to Gatlingurg no towing (about 600 mi) and got about 11-11.5 mpg. Not a heavy camper but I guess the big block could barely tell the difference.
Pulled the same camper to Destin last year with my Dakota that gets about 15-16 mpg without towing and it got 9 mpg towing the camper. That was not fun, probably not real smart either, even though the Dakota has a 4.10 rear end. With an 18 gallon tank, we couldn't drive 2.5 hrs without stopping for gas. Not much of a comparison - a 4.7 vs a 7.4 but kinda interesting how significantly it changed towing and not/towing.
Towing our camper is primarily why we got the Suburban - tows and rides great. I will probably only put about 3,000 miles per year on it (and maybe 30% of the time it will be towing) so it would take me a long time to pay for a diesel swap. Still plan to go that route if my engine dies but I'll probably be about 120 years old by then - but no 2 ways about it, a Cummins Suburban is just cool.