I think you may have answered your own question when you compare prices of a 3 years old truck vs. a new truck.
Here's few that come to mind.
New truck Pro's:
1. Truck has not been abused
2. All wear items are new (i.e. brakes, shocks)
3. In the last 3 years diesel trucks have really improved with emissions with the introduction of DEF.
4. Full warranty
5. Less unknowns and surprises
New truck Cons:
1. Cost's and depreciation once you drive it off the lot
Used truck Pro's:
1. Take advantage of depreciation (May or may not be an advantage)
2. Truck may have a few dings/scratches so now worries about getting the first ding.
Used truck Cons:
1. Unknown surprises
2. Unknown maintenance intervals
3. Truck may or may not have been abused
4. Truck may not have DEF equipped emissions
5. May need to consider purchasing an extended warranty
6. At the end of the day can you trust it? Did you really do your homework.
For me the only reason to consider purchasing a used diesel truck is to buy one with pre DPF emission equipped, otherwise your better off buying new if your funds will allow.
We recently purchased another used diesel truck (pre DPF) and they are hard to find one in decent shape and that hasn't had the snot beat out of it. Some of these trucks I wouldn't trust pulling a travel trailer long distances.
Good luck... :B