John,

Can you surface mount the box, or is this something you would rather not do...? If you use a WP Box, all you need is a hole that the wire will pass through, the box hole size is 1/2 inch threaded. Another thought is you could mount the box under the floor and run the wiring in conduit, through the frame. If you want just the flush receptacle cover, you will need to make a larger hole to accommodate the box and receptacle.

If you are talking about the "Power Feed" TO the trailer, that is a whole different scenario........ There are some very small, very quiet, and inexpensive generators on the market, that will fill 90% of your power needs, and you don't have to worry about "plugging" in. That is, if you are just going to use lights and chargers.....if you are running a machine shop in there, I might suggest a 3 phase hook-up........