I have a '99 F-550 with the 7.3, automatic. It has a 5-ton rated, Crysteel Triple-Tipper dump body, standard cab with Backpack to all of my tools.
I, almost daily, pull an equipment trailer and a mini-excavator, which together are about 10,000lbs.
And, I pretty routinely pick up a load of stone, too at another 10,000lbs.
That means, that in addition to the weight of my truck, dump body, and tools (about 11,300lbs), I pretty routinely add another 10,000 in the dump body along with 10,000lbs pulled behind.
GVWR of about: 31,000...and the truck does just fine. The only modification that I have is a slightly beefed up transmission and an extra, transmission cooler.
That being said, even before I had the transmission rebuilt, I never had overheating issues.
In 4x4 low, the truck never had problems pulling all of that up steep, narrow, mountain roads.
And, on highways, at just 21,300 GVWR (trailer but no load in the dump body) with just keeping it in standard, 2x4 drive, she'll slow down to 55mph or even 45mph on very steep sections, but no overheating and no problems.
Otherwise, she happily cruises at 65-75mph on flat highways at 21,300GVWR.
Loaded or unloaded, I have NEVER gotten better than about 8.7mpg...
...guessing the extra weight and wind resistance of the dump body, frame, dually, etc aren't doing me any favors there.
I have thought about adding a rear locker for the more treacherous, mountain roads and a basic chip to help with maintaining highway speed, but have not done enough research on what would be best...