Driving safety is the most important factor while renting an RV. You should rent an RV that you are comfortable to drive and maneuver to your destination.
The two factors that affect driving safety are the length of the RV and its weight. The longer the RV the more difficult it will be to maneuver, the heavier the RV the more skilled the driver will need to be.
If you are a first time RV renter then select a smaller RV (24-28 feet) that will automatically have less weight. Also select an easy destination that is less than 400 miles from your home.
Once you are comfortable driving an RV then you can plan longer trips and perhaps rent longer RVs should you need them.