Dealers claim they get "Specials" from the manufacturers for RV shows but my experience is that you always get a better deal away from the shows. There are a lot of folks buying at the show and the dealer is less willing to make a deal work than when you are the only one on their lot on a rainy Wednesday at the end of the month.
In reality, the best time to buy is when you find the rig that you want. Most dealers will work with you to get to where you need to go. Emailing them ahead of time working one dealer against another generally gets you a good deal but you need to know where to start. 25-35% off MSRP is a good place to begin from but often the MSRP is a bogus number. Some online sleuthing may help you get a good MSRP from the manufacturer. Of course all this advice is good for new RVs. On used, it is tough to start but after lots of comparison shopping and using the NADA pricing as a guide you won't come out too badly.
There are lots of forces at work as to when to buy. Fall is when you might get a good price but inventory is down so it may be hard to find the one that you want. In Spring the inventory is up but so are prices. For these reasons I think the best time to buy is when you find the one you want.
I am NOT a mechanic although I do play one in my garage!