There's a federal law that requires you to either swear that you know the odometer reading is accurate, that the odometer has rolled over, or that the true odometer reading is unknown. This disclosure is made on every vehicle sale, and there's a standard form.
If you can't swear to the first two, you swear to the last one.
I wouldn't assume that a low mileage fleet vehicle is necessarily beat. Maybe it was assigned to a short route (There's a FedEx route on Lopez Island in Washington, half his day is spent on the ferry), maybe it is being phased out early due to a change to electric. There's a lot of reasons FedEx might sell a low mileage vehicle, and if you have a legal document from a major corporation stating that the mileage is accurate, most people would trust that.