I have used tennis balls to cover the gutter extensions before putting on a cover. Cut an "X" and fit the ball over the extension---may have to experiment with the size of the "X" and the fit of your extension. Can't take credit for this---read about it here from someone else!
Our gutters have not been hurt in the ten years we've been covering them. But, I see that newer RV's have gutter extensions that stick out further than ever. Using something like the noodle sure wouldn't hurt anything,