I have owned an RV (small one -- class C) and towed a car.
Some auto trans cars can be towed with 4 wheels on the ground and a hitch which usually mounts on the car.
Some can not be towed that way stock, but can be modified to have a transmission pump added so they are OK, and some can not be towed 4 down at all. Some makes are more popular with RV owners because they can be towed 4 down.
We had a Toyota Highlander, which can not be towed, and I opted for a tow dolly rather than a modification to the car which would have voided the warranty.
A tow dolly is a major PITA compared to towing 4 down. The biggest problem is that the wheels of the tow dolly are wider than even the RV, so corners become difficult. Hooking up takes much longer than when towing 4 down. And, best of all, what to do with the tow dolly once you park the RV in a small spot becomes an issue. Tow dollies weigh 500-600 lbs, and while they are well-balanced to allow you to move them by hand on level ground, even a slight slope gives them a mind of their own and a lot of mass to back that up. I added a trailer hitch to the Highlander just so I could move the tow dolly with it.
Check RV.net for more information and opinions. If you are even thinking of getting an RV some time in the future, think about getting at least one car that can be towed without a dolly.