The customer is who decides who does the normal routine service work
The dealer will want it and will try to get it, but i do not think a covered warranty claim hinges on who does the service work on the truck.
We have customers that service their own trucks but we still do warranty claims for them as long as their techs don't damage the trucks original state, the tech services the truck as per the recommended maintenance specs,
and the failure cant be traced back to anything that the non-factory tech did or did not do.
it only can be covered by a qualified manufacturer dealership though.
I'm not sure exactly what the consumers rights are in the law when it pertains to warranties and dealerships, but i think a dealership can deny working on a truck if they choose to do so. I believe the factory is who is responsible for covering the warranty and any factory supported dealership can do it, but I don't think it would be in their best interest to turn down doing it. Being that the dealership is a representative of the factory they may be obligated to cover warranty.

A wise customer would use the dealership for his routine service during the warranty period, and hopefully from there on out. ;o)