Ford truck chassis are serviced at Ford truck dealers. In markets that support 2-5 dealer, likely one will be a truck dealer, might even be a truck-only dealer. In larger markets, major urban areas, there may be two or more truck dealers, likely to be truck-only. In rural areas, where towns with dealers might be 50-200 miles apart, all the dealers will sell and service trucks, may represent more than half their business.
You will find the same for motorhomes on Chevy chassis, maybe fewer because Chevy has been off and on selling larger trucks, Ford never quit. All GMC dealers should service Chevy van chassis.
Situation with Sprinter is trickier. All Freightliner dealers will service any Sprinter. Some M-B dealers will service the Sprinter and other M-B trucks, but most are car only. Dodge dealers that sold the Sprinter as a Dodge should be able to service the model years sold, but as with Ford and Chevy, not all Dodge dealers were truck dealers. What you might be looking for today to service a Dodge Sprinter motorhome will say they are a RamTruck dealer, but no guarantee, as there are new Ram dealerships that were never Sprinter equipped.