rjw, welcome to irv2 and the Newmar forum.
Take a look under slide when it closed. You will have four rollers under front edge of slide one of them may have lost a roller nut on a bracket thats causing the slide to track the slide uneven when closing or a roller is cracked and needs to be replaced. You can order a roller and replace it you need slim hands while laying on floor.
Let us know what you find, enjoy the forums and post often.