I'd sure run a check on the VIN number for any repairs or higher ODO mileage statements or indications. Also check for codes. Yes, the IP cluster can be switched out using one from a crash totaled junk vehicle or stolen vehicle with a lower mileage on the ODO. Illegal but it's being done to some vehicles that have very high mileage to increase their sales value. A cheap cheat!
Truck may have been use comercially or hauled RV's etc around the country. Should not be oloose at 145K. Not sure if Michigan will still let you do a title search of the truck's past ownerships like they used to. It would be a big help if it was always titled in Michigan.
11.4 mpg is way too low and either there's a problem or the DPF is partially plugged or the truck has been running a long time at idle. YES, reset the DIC mpg's and go immediately out on the highway or reset it when on the highway and then set the cruise at 60 or 65 mph and and drive 10+ miles on level ground and see what is says. The GM DIC mpg are very accurate if the original size tires are on the truck. Should read 17-20 for a 2008 at that speed if fully warmed up. City driving is a can of worms for mileage over say 10-15 miles due to what speedlimits, how many starts and stops, and how many red lights sitting at idle.
Our 04.5 D/A CC LB 4X4 gets 18+ to 23 at 60-65 mph on level ground with calm winds with the 2 of us and our Siberian Husky and the 5th wheel hitch and 2 aux group 31 batteries in the bed and always has. If we were home, I'd sure let you drive ours for a comparison. Ours, purchased new, with that many miles is still very tight and drives just like it did when new and nothing's and been repaired or replaced but the front brake pads last year, the tires - 3rd set on including the worn bald Bridgestones at under 30K and the new set of Michelins now, and 2 light bulbs that burned out.
There may be a reason the price is SO right!