I guess I have been lucky with the few shops that I have used, but this is my procedure, and it seems to work.
I call a repair shop and schedule an appointment. I drop off the motorhome the NIGHT before(I don't let it sit in their lot for 2 weeks) so that it is immediately available to the service people when they arrive at work the next day. I also let the service mgr know that I will call in late morning to see how things are going(if he doesn't call me first). If he doesn't, I call before NOON to see how things are going(usually the tech is out to lunch until 1 pm and they can't give me an update until he returns).
If parts are needed that they don't have, depending on my need to have my MH back, I ask them to have the part shipped Next Day, 2nd Day, or 3rd Day(I am willing to pay the extra shipping charges)if necessary. I also let the service mgr know that I will be calling back in late afternoon(before 5 pm)to get another update on progress. I have always got my MH back in a very timely manner(2 or 3 days).
I can only assume that they know I will be checking on the progress at least a couple of times each day, so they don't put me off for days at a time. This procedure has worked good so far.