I'd say about 150% that it is inaccurate. That works out at 3083 miles a year and no one except maybe the federal government would spend over $100,000 or more on a new bus and not drive it way more than that. Maybe it was in storage in Antarctica or something.
I'd bet the speedometer was replaced for some reason. Perhaps the actual mileage is recorded in the computer--might be worth a look. Also, where did the seller get the bus? Could it have been flood damaged and repaired? Who's name is on the title and is the title clean?
It could be a killer deal but you best do a lot of homework (and drive the bus) before you buy it.