I would hope that the trailer idea is done using the MX battery through an inverter as the energy source for the Airstream (good stuff, check out Thor Industries, stock symbol THO).
If they can show not only that the trailer can be towed by the car - but also powered by the car for hours on end, that is a good thing. Tesla needs an onboard auxiliary inverter to power a 120V circuit with a 1000W+ inverter, if possible. Much like EVExtend.com did for Volts and Leafs. At an RV park, you can plug in a UMC and charge the car and also run power to your trailer simultaneously.