I have a little over a year of towing experience now with a Model X (100D) and an Airstream (FC20). The numbers do not work out to lend themselves to anything like the imagined "tow a trailer with a battery that will give you 500miles of range".
-Range is about cut in half (more if traversing hills or in bad weather)
-a second 100kWh battery (which would be largest and most expensive if we stay in the Tesla realm for the moment) essentially would almost get you back to car-alone range, BUT you would have a 5000lb little home with you wherever that range took you.
I think the idea is cool, but the rocket equation makes it not practical-and then generator ideas come into play, etc.
break break, what about a smaller ambition (for people who want a RV trailer)?
1) Concept 1, rip out the factory lead acid trailer batteries, and put in the smallest Tesla battery-maybe a used one?- if you had 50kWh of power in your trailer, you could run it like a house while boon docking (full A/C, etc). If you had a supercharging capability, you would be able to recharge it really fast.
2) Concept 2, go light with the trailer, but somehow tap into the Tow vehicle's battery with full current demand. it would reduce the radius you could go away from a supercharger to camp, but why even drag extra batteries in the trailer, when you have a massive 100kWh battery sitting right in your tow vehicle and able to be supercharged? My real problem is that I can't get to that energy except to move the vehicle. (not enough current capacity through the 12V trailer port to do anything except trickle charge trailer batteries, certainly not enough to run an inverter, etc.
3) Concept 3, I like the idea of regenerative brakes on the trailer, but I don't know how much weight they would add. And if they only charged the trailer batteries I am not sure how useful that would be. Still, it is overall energy on the system and if something like Prius regeneration brakes could be recycled from wrecks, maybe that would be cost effective for a science project. As currently plumbed though, excess energy in a trailer would present itself like a full battery to the Tesla's 12V trailer connector, and all that would do is stop the on board Tesla 12v battery from flowing current to the trailer (might charge the onboard tesla 12v if it needed it, but I am not sure of that-certainly would not feed back to the larger battery system -so you would have to chop into the guts of the Tesla for very very small returns.)