Very simple to fix. I've done it a dozen times, I was a coach tech at a RV dealership. There is a thin plywood under the headliner, and it's called luan.
Find the 3M spray glue that is white, or clear. Don't get any other, I warn you. They didn't work for me, and it cost me money by having the coach come back and I had to fix it for free.
Start at the edge of the sag, about three or four inches inboard. Push up on the material, and use a very sharp, New no. 11 xacto blade and cut a slice about 1/2 to 3/4 inch. Put the spray tube in the slot and spray around to cover as much as you can. Let it hang for just a few, and stick it to the ceiling. And being careful to keep the material from bunching up as you go. It only takes a minute of thought to keep it smooth as you glue it back up. Good luck, it's really not hard and looks factory when done.
Just remember to spin the spray tube around in the slit to get the glue around, in a circle. Then the same again in another spot, until it's all glued back in place. You might have to kinda push the headliner to keep it from being stretched, and go back in the same.place it was before it sagged.