I would use 8 or 10ga wire and a 25 amp fuse. The suggestion above to use a rearward compartment to keep it out of the weather is a really good idea. If that is not possible I would mount behind the bumper (between bumper and TT rear end) to give it as much protection as possible for stuff hitting it. If you have a rearward compartment to mount it you can still run the wire under the trailer (easy) and then just drill a small hole in the floor to run the wire up into the compartment. Seal the hole with either expanding foam spray or semi-flexible Dicor.