To asnwer your original question. You probably can not find mileage on the odometer that is true. Sure, it might be close, but you really don't know. I picked up a 1009 from Warthog a week ago that has over 76,000 on the meter. Warthog did a Carnac report and the truck was said to have been last used in 2005 with 75,000 on the meter.
That made me think the thing had been driven a bunch. However, the pedals and seats don't show near that many wear miles. Then I started driving the truck. 85 mph indicated is 65 gps actual. How long has the mileage been counting 25% more miles than it actually covered? The only thing I can say with any basis of fact is this. My truck probably doesn't have more than 76,000 miles on it.
At least with the CUCV series, both Dodge and Chevy. The speedometer is hard to change out. All other MV's the speedo can be swapped in about 3 minutes. All that means is the CUCV mileage will probably be more close to reality than the other trucks.
As suggested in previous post. Look at the truck and not the gauges. If it looks good, get it.