If an RV is well maintained then the age of the RV does not matter so much. Nevertheless if you have a tight schedule and less margin of error e.g. if you are driving 1000 miles to Yellowstone National Park and back in 5 days then you do not have any margin of error. In such case perhaps you should rent a newer RV or rent an older RV with very positive reviews.
On the other hand if you are driving only 200 miles for a weekend trip then you can consider an older RV if there is adequate price savings.