I think the best time to buy depends on what part of the country you are actually in. The big dealers in Florida seem to make some pretty good deals starting in January when all the snowbirds get down here.
What you may consider is the possibility of needing warranty work after purchasing your unit and where that work will be performed.
We got a deal from a huge Florida dealer that we couldn't pass up. Didn't need any repairs or adjustments the first 3 or 4 months. However 6 months later we had about 4 or 5 things that needed attention that were warranty issues. We thought we were very fortunate because where we were camped up north there was a dealer a half mile away.
As soon as they found out we didn't purchase our unit at their dealership it was like pulling teeth to get an appointment with them. After weeks and weeks of waiting we finally got in. They spent about an hour looking things over, said they didn't have some parts, and didn't fix anything!
We made an appointment at our original selling dealer for when we returned to Florida, which was about 10 months after purchasing the unit, and they fixed everything. Believe me when I tell you that warranty issues on new RV's don't work like warranties on new cars/trucks.