If you only travel downhill, sure it's free.
I regenerative braking efficiency is more than 30%, not sure how much, but that's efficiency of converting work your RV engine produced to begin with. So, for example, if your RV had to produce an extra 10kW to account for the extra force applied by regenerative braking on a flat towed Leaf, your leaf would only get 3kW (or whatever it would be based on real efficiency). When you have to use an ICE to pull it, no it's not more efficient than an ICE. The efficiencies multiply: %ICE*%RB*%EV = a lot less.
Yah, flat towing makes a lot of sense on a typical manual car. Unfortunately (or fortunately, depending on how you look at it), the Leaf is not typical and it doesn't have a manual transmission. Because of the differences, there are a lot of unknowns. For example, the regenerative braking feature is meant to be used intermittently or for relatively short periods of time. It may or may not be capable of running constantly for an hour. Same goes for back EMF. Components are easy to understand but the way components interact not so much. Really, the only way to know is to try. Nissan isn't going to give instructions on it and it would probably be very difficult to get all of the details from them.
I say go for it! Tell us what happens. I'm very open when it comes to experimenting with other people's property