John....

....another thing you want to think about....I presume the trailer
will be parked out behind your house, or in the driveway, or
whatever....when you are charging your stuff, or working on
something inside the trailer.

Get a 8" grounding rod, and pound it into the ground next to where
the trailer is parked....hook a 6' wire to it, with a big alligator clip....
yep....ground that dude when your running elec. power in there,
and use a GFI recepticle on the house end. With that set-up....the
slightest ground fault (or short) will trip the safety....and cut the
power off instantly.