Inside the RV, the top of the slide (its roof) might have something on it at the inner end that hits the RV wall where you can't see it behind the trim (forget the right word for that) so whatever it is gets jammed and stops the top of the slide going out farther.
Is there any slack in the awning so it droops a little when out, or is it bar straight like it would be to hold the top of the slide back?
You can compare how vertical the inside of the slide is wrt the wall of the RV, and watch it going out for when the top stops and the bottom keeps going out. Stop it there and check outside. Is that when the awning goes tight and stops unfurling?