This has probably already been discussed in vast detail elsewhere, but it's the first time I thought of such an idea today. We've all seen large campervans and RV's towing smaller cars to be used whilst on vacation. What if that car was electric, and there was a "tow setting" Where the software allowed gentle regenerative braking whilst being towed, so when you reach your destination, you have a full battery?
The extra drag would be at the expense of more diesel/gas in the RV, but the software would detect when the battery was fully charged and allow the car being towed to freewheel once it's full. Obviously the car would be towed extremely close to the RV, so aerodynamics would not matter anyway. Would this be a possibility? I think it's a great idea if it works.
I would have thought it could be done with any EV with regenerative braking, without software mods. Current software should cut off regenerative braking when the battery reaches full charge, otherwise battery could be damaged. Most EVs are rear wheel or four wheel drive so the front wheels can be raised on a towing dolly. Obviously the electrics would have to be switched on.
Just put the car in drive and start towing should be possible, in theory. But after a quick search on flat towing a Tesla, the company doesn't support it. It is pointed out that regen charging while towing would be inefficient in discussions, and doing so would put additional strain on the towing equipment and vehicle frames than what an already heavy car would do.
It is also unnecessary, at least in the US. RVs make use of 50 amp power outlets while parked, so campgrounds will be well equipped with them. The Tesla portable EVSE(charge cord) has multiple adapters available for different outlets, and the car is supplied with the one for 15 amp home outlets and the one for 50 amp RV ones.
Early adventurous Tesla owners actually made use of RV campgrounds as charging spots. For other BEVs, you can get a corded level 2 EVSE with that RV plug.
For those doing more off the grid RVing, the RV itself will have a sizable generator. Heck, Volvo is planning on offering a diesel powered fuel cell to replace such generators on sleeper trucks. So higher efficiency and lower emissions when that comes to past. Ideally, you would charge the car from the generator/fuel cell while the car is being towed in order to have it fully charged on arrival, and to avoid some load juggling while powering the RV and charging. That will require some software updates to the car.
Interesting. Imagine in the future a situation where it's possible to "latch on" to a heavy vehicle such as a semi truck, similar to how planes refuel whilst in flight, and you're able to be towed and have free regen whilst you take a break. The autopilot software would be able to keep the car straight and detect when the truck is slowing down. No doubt such scenarios will exist in the near future.