The coach has urethane clear coat on it and not lacquer. If you are seeing fiberglass cracking or what some folks call 'checking,' the only permanent fix is to replace the sidewall/s. Very expensive. Sanding, priming, repainting and clear coating the wall will not fix it. It will reappear. I had the fiberglass checking on a 2002 Winnebago Journey and had a full body paint job done at great expense. It looked wonderful but the small cracks in the glass started to reappear in about 3 years.
I would strongly suggest you look for another rig. There are just too many good used RV's out there to take this problem on unless you can get it really cheap and live with it.