Our Timber Ridge has a 21ft awning, and sagged from day 1!!! I installed a center support and don't regret it. Our previous Jayco had an 18ft awning and I had a center support on it also.
We travel back roads, Forest Service roads, one lane dirt/rock roads and the supports do a great job of preventing further sag, and bouncing in the center of the awning.
Highly recommended for almost any awning. Newest ones seem to be more flexible and prone to sagging.