We have one truck that definitely has over 1,300,000 (and may actually have
2,300,000) miles on it (1979 International S1700, DT466), and another that has turned over so many times that nobody has the vaguest idea of the actual miles (1972 F-800, 361FT). And, I guarantee the miles are several hundred percent harder that 760 mostly-highway miles/day. I'm afraid to even think about how many engine hours are on the trucks (especially the IH).
We have one truck (2001 F-550, Jerr-Dan rollback deck) that will regularly go a week without being turned off. The trucks see city driving, some brutal overloading (I had a Chevy 3500 CC/DRW on the 550 once, with a slide-in in the Chevy...I'll eat my toolbox if I had less than 24,000lbs on that truck), curb hits, and lots of idling and no-load 1500-2000RPM running (for the PTO's). The 550 is the lowest-mileage truck, with 339,000 as of yesterday. It just had its first major breakdown, since Nate was just barely able to limp it back to the shop with what turned out to be a bad IP. Other than that, its only non-deer-related hook call was the alternator (not the factory one) seizing due to what was probably a manufacturing flaw.
My usual truck is a 1986 F-350...as of last month, it was in service for over 20 years (and Chuck still has the reciept from Ford for the chassis-cab). It has over 600,000 miles on it, and the 6.9 IDI still has the factory pistons, valves, bearings, and I think head gaskets. Chuck says he'll do a full frame-off restoration on it at 1,000,000 miles.