Whatever you do- don't go to Volvo or Cummins. Honestly- we've had no dealings with Dallas Cummins- but Onan, which is the same freaking people just different sides of the shop- were TERRIBLE!
Why do you need the Dallas area? What kind of repair/maintenance? What motor? We've had bad experiences at Volvo in Dallas, Fort Worth AND Houston- so we stay away from Texas now.
A good mechanic is worth his weight in gold.
My "finds" for good shops are Cummins in Phoenix/Avondale, Cummins in New Jersey (Jersey City Area- though we have very limited dealings with them), Aber's Truck Center in Ashland, Ohio, and, for a great alignment, Laserline in Twin Falls, Idaho (mobile alignment).
I have a few more "golden" shops but I can't think of them right now.