Standard advice is to get a pro truck mechanic to inspect the truck parts and give written estimates for any repairs needed, parts and labor. RV tires become unsafe sitting parked for 5 years. They should be replaced if sidewalls have cracks deeper than 1/16 inch regardless of appearance and tread depth. Brake fluid should be changed every couple of years and brake system serviced as needed. Engine cooling system including all belts and hoses should be serviced as needed. Buy a popular brand of motorhome where maker is still in business.
Have an RV service inspect for water damage and function of 12 volt DC and 110 volt AC electrical system including, batteries and converter charger RV generator, roof AC, furnace, fridge, water heater. Inspect awning fabric and mechanical parts. Whatever you buy, try it out at a local RV park or in your driveway before embarking on a real trip.