The fridge and roof AC unit on our 2004 rig died at about the 10 year mark. Make sure that RV generator, all appliances, fresh water pump, water heater, dash and roof AC work properly. If tires are older than 5-6 years old, (per date codes on tires), replace them with proper load range and name brand tires. Have entire brake system inspected and have brake fluid flushed and replaced, replace front brake flex lines. Check awning fabric for deterioration and mechanical opration. If there is mold smell or discoloration of interior especially in the front overhead area, don't buy it.
Get independant estimates from an RV repair service for anything needing repairs, upgrades or replacements and consider these when deciding to buy or not.
Oops! forgot to mention have all belts and hoses and engine cooling system checked and serviced as needed. Have transmission checked.