The mileage may indicate a lot of days of use, which leads to interior wear. Upholstery, cabinets (look around handles and hinges), flooring, etc.
As for the mechanical parts, ask if there are maintenance records or if they can get them (they usually will call the prior owner for you and ask, if they are not with the coach). Good-to-excellent maintenance records, e.g. oil & filter changes, chassis lubes, routine repairs, will generally indicate the rig was well cared for. Lacking them, I would assume that ALL filters and fluids (engine, fuel system, transmission, air intake, cooling system, generator, etc) need to be changed and that is a hefty chuck of change on a diesel. Figure $1500 or so. See if you can get that included in the deal. NOT just an oil change, which they usually do anyway. Diesels have a lot of filters and they are often expensive buggers.
And don't forget to check tire AGE as well as wear. A 99 may well have original tires (yikes!) or may have already had new ones installed (good deal).