The GM diesel trucks have had problems with regens and with DEF usage that are attributable to failing NOx sensors. I would not be at all surprised if the Ford diesel engines had similar problems. What makes it difficult is that the sensor may appear to be functioning properly and the dealer's mechanics are operating in the dark and trying to guess what the source of the problem is with the very little information they get from the computer codes.
Suggest checking for service bulletins for 2011 and 2012 trucks relating to regens, NOx sensors, and DEF usage. Often when trucks leave the factory it may be months before they are sold and in the interim service bulletins may be issued by the manufacturer.
The dealer is supposed to check that there are no applicable service bulletins for a given vehicle and to apply the necessary fixes before the truck leaves the lot. There is not a lot of incentive for them to do this prep work when they get reimbursed only when the truck comes in later for work covered by the manufacturer's warranty.
For use in hilly terrain a heavy pumper truck should have the diesel engine. I did mountain rescue at one point in my life and the sherrif's department had an old Dodge Powerwagon that on mountain grades could only make 45 MPH at best. We would have the siren going and have to be in the slow lane to let other cars and trucks go past us.