I think the best time is when you have a motivated seller. When something happens that makes a responsible owner need to sell the RV. For example Grandpa passes away and Grandma doesn't want to continue travelling. Or Frank has a great RV but his job just moved him to Hawaii, or laid him off, or the new unexpected child is coming.
The people who realize right away that they need to sell are the best to buy from. They are realists. They aren't letting the rig sit for 2 years without maintenance, hoping things change. They recognize the need to sell and they are motivated to make it happen.
I purchased my current MH in May one year because the husband passed on and the wife could not make the payments on her salary alone. She recognized the problem, contacted the bank and advised she would try to sell by a certain date. If she couldn't get it sold she would simply return it to them. They were good in working with her to sell, and me to buy. They gave her a new note for the amount that she was underwater for, allowing her to avoid any hits to her credit report. Everyone in the deal was working to make things go smoothly.
I wasn't really wanting to buy in May, I really wanted to wait until September. But I had been following the prices and availability of the models I wanted for quite some time and knew a deal when I saw it. And I had my ducks lined up to buy when I found it.
If you are sure you want to buy from a dealer only, then find one that can complete the deal without moving his lips. Generally speaking if his lips move, he's telling some type of lie. My appologies to the one honest RV salesman, wherever he is.
Ken