Some places will not let you walk through their service departments, at least not in the areas where the work is actually being performed. It's a liability issue.
But you may be able to observe some keys signs of a well run operation -- the place is clean and organized, walk and fire lanes are clearly marked and observed, spills and debris are quickly cleared, and the staff uses ground guides when moving equipment into and out of the work bays.
There are a couple of websites that permit users to add reviews to RV-friendly services, including RV sales. Here's one:
www.rvservicereviews.com