bought a used r.v. how to determine actual mileage?

The ABS sensor was not the problem, it was the output speed sensor. Seeing as the vehicle obviously was driven at least some distance with the sensor not working, there are more miles on the vehicle than are showing. There is no way to tell what the actual mileage is, and the vehicle is now considered to be TMU (true mileage unknown)

At 174,000+ miles, the fact that the vehicle is TMU will not make that big a difference