Newmars are noted as being very good RVs. It looks like it has no slides, which should be a plus in something this elderly. Replace the tires, and clean and wax the exterior. Go over the interior with lots of mold inhibiting products (bleach is cheap). Open the vents and windows and put a fan in it to ventilate it. Check the cabinets for delamination. Use a lot of fabreze or something like that on the fabrics, maybe even get a carpet cleaner in to "wash" the carpet to get rid of molds. Replace that mattress too
Let us know how it turns out with some before and after pictures.
