When we stayed in the Treehouses, the shuttle was running at pretty consistent 10 minute intervals. We used it a couple of times and could often see them driving through the property while at the pool, walking around, etc.
So you shouldn't have to wait long for the shuttle. I don't think the time of year would matter since that shuttle isn't going to be anywhere near full on any trip.
The shuttle takes you to either The Grandstand or The Springs stop at SSR where you need to board a separate bus to hit the parks. That's where your wait time will vary more.
Staying at the treehouses should only add 10-15 minutes to your journey, but then you are subject to the usual variations in theme park bus service.
We tend to have our own car available to help avoid all of those waits. But be warned each Treehouse only has one parking space available. The Treehouse area is designated as protected wetlands so they had to make do with limited parking permitted by the original design.
If you have two or more vehicles in your group they ask that you park extras in the nearby lot at The Grandstand.
...in advance!
Bill
