Newbie - '90 Holiday Rambler Limited DP Sagging Ceiling

If it's just the liner material, it may be simply age causing the adhesive holding it up to degrade. One potential easy and low-tech way of fixing that up is to put some staples in it (with a T-50 stapler); depending on the material, this may even be largely invisible. Another option might be a series of battens.

If the whole structure is sagging, you really don't want this motorhome.