We tried the rental route when we first started RVing. As many have mentioned it's iffy at best. Having a reservation doesn't necessarily guarantee a car will be there when you arrive. Many times cars come in late, have been damaged, or need service before they can be put back on the road. If you're going at peak travel times there's a good chance you might have to wait until a car becomes available even though you have a reservation.
A couple other drawbacks are rental car location in relation to your destination, and cost. We've had the nearest rental agency be as far as 80 miles from our destination. In addition even though we've made reservations far in advance the size car we reserved wasn't available when we showed up. In some cases we were told we could either wait until a car became available or upgrade to a more expensive vehicle. In most cases we were able to get a car without additional cost, but it took serious negotiating and time out of our schedule.
We quickly gave up on the rental option and started towing our Buick on a dolly. After several years we purchased a Jeep Wrangler to tow 4 down. It's been a much better experience taking our own vehicle along. No hassle, our own schedule, and our own route.