I too would have serious reservations about any motorhome that's been sitting for five years. It's not only a case of replacing hoses and belts, but the wheel bearings, differential, steering joints seals, alternator, pumps and compressors have not worked. The 5.9 is not an engine intended for rebuilding like the C8.3 so the engine should be inspected by a competent Cummins technician.
5 years is a long time for the genset not to have worked, so expect major repairs and overhaul, roof air conditioners, refrigerator cooling unit (major expense), propane system and perhaps the fuel tank containing sediment, water and/or fungus.
The diesel fuel in the fuel tank should be drained and replaced with fresh fuel, change all fuel filters, air filter and hydraulic fluids replaced. Transmission oil should be changed as well as differential oil changed.
Hydraulic jack system should be flushed and checked to make sure it still works.
Awnings should be checked for mold and rot and the holding tanks should be checked, fresh water tank sanitized and water pump checked.
Hot water heater should be flushed and having been out of service for 5 years might have to be replaced.
Has the interior of the coach been checked for leaks? After 5 years of sitting, plus it's total life, caulk and seals need to be checked and perhaps replaced.
Hoses and belts could total about $400 - $500 but I would look at much more preventative maintenance.
All batteries, coach and chassis should be replaced.
Oh yes, pull the wheels and check the brakes and most important after 5 years, the tires absolutely must be replaced. Though the tires may not show wear, the degradation of the rubber requires that tires be replaced 5 years after the tire manufacture date.
Does the coach have an air suspension and/or braking system?
Air tanks should be inspected for corrosion from water condensation, the entire air system should be inspected for leaks, perhaps change hoses, replace air dryer filters, and adjust ride height valves, and ensure the slack adjusters are functioning.