I bought parts over the phone from the apu center mentioned in the OP. They were easy to deal with. I ordered a bottom case half and service door for my April '06 Tripac. They sell an aftermarket piece that gets the job done for less than half the cost of the original from Thermo King. It doesn't come with the insulation like the original had. Which is probably a good thing, since that's half the reason my old one rotted out with rust (holding water). I've never visited in person, so can't speak of their shop service.
Big Rig was fantastic. Dude who worked on my Apu was very thorough. Great place. I'll try the Apu Center next time.