I would suggest sanding off the popcorn (if not too pronounced) and using a good quality vinyl wallpaper to cover all the slideout walls. You should be able to find something that will contrast nicely with the surrounding area.
I believe that the "wallpaper" the RV manufacturers use is bonded to the paneling at the factory where it is made and that it is not available as a wallpaper as such. Attempting to replace the paneling , assuming you could get it, might lead to more problems than you already have. I would do so only as a last resort.
Happy Camping! ///// Richard D.
2006 4x4 Ford 250 SD / 2007 Flagstaff 827 FLS
One very patient wife and one furry child who travels with us. Forty-two years of trailering and camping, and I still have a blast.