If I were going to build a bug out trailer, I'd make it a sleeper and definitely sacrifice some space on the interior so that it could be well insulated. In doing so, as long as you can access it easily, you've got a warm (relatively speaking) and dry place to sleep pre-made.

And whilst I'm dreaming, I'd make the gear storage space pull out from the interior which would then be open space to bed down in much like a drawer. Something like this, but without the openly accessible sides on the outside...