One can do a search for the name of the RV place and limit it to sites like rv.net, irv, and other forums to find info. Usually you will get complaints, since happy customers don't tend to post good in general.
The funny thing is that the same dealer can have a top notch salesperson and one of a horror story... and both sitting next to each other in the showroom. For example, the same dealer with a salesperson who helped a friend of mine and his wife get a very good, usable class "C" for quite an excellent price, I ended up with a different salesperson who was definitely nowhere as good.