The 2013 250 DSL is coming up on 95 thousand miles. What is the recommended
mileage to change out the belt? I pull a 15 thousand pound 5th wheel and spend a good part of my time running around the outback in some of the nastiest real estate possible.

 

If you're thinking about it, just do it. They're not expensive, but if it does break, you're dead in the water. Kind of like a battery - for what it does, I stick a new one in every three years.

 

Replace it and then carry your old belt as a spare. You will breath easier and worry less.

 

as above.

and replace all Idlers pulley's... keep old ones just in case..
I changed mine last Fall.. all Motorcraft parts. Rockauto.

at 136,000 miles 2 idlers bearing were dry of grease. I had just purchased the truck