I use Jiffy Lube for oil changes and such without any problems. Their in-floor changing areas mean they aren't concerned by lifting a heavy vehicle.
For anything else it goes to a Ford Truck Dealer or an equivalent. I thing the problem at most dealers is the lack of lifts that can handle the weight of an RV. My Ford truck dealer even attacked a tricky issue with the rear lights that involved portions of the wiring added to the RV in manufacture.
Maybe you should try GMC truck dealers.
After warrenty try to find a good large-truck shop for the mechanical work. I have given up on an RV dealer to do repairs for other areas. I just tinker with them myself until the problem gets solved. When I am too old to climb on the RV roof, I plan to look for other travel alternatives.