Although i am a newbie im a retired mechanic with experience in the drive train of RV and big trucks. That being said i need help with a new head liner.The one in it now is leather and is sagging,had small leak,roof is new dicor.Does the head liner come out?Is it put in before the interior walls,ie.do i have to rebuild the interior to get it out?Any help would be greatly appreciated,thanks to all.