When I sold RV's we used fridges all the time for keeping cold bottles of water scattered around the lot for shoppers as they went thru the units and sometimes cookies also. I know we never used a microwave in one though but who knows someone may have kept their leftovers in the trailer also! It was also very common for employees to use campers on weekends for fun so maybe an employee had used it for a night or two and just didn't clean up after themselves. Despite what a lot of folks think dealership employees in the RV industry like to camp.
If it was a rental it would have had to be plated and titled for insurance reasons so if it was sold new then I highly doubt it was a rental.