Locating the controller inside the trailer and using the original wires that can be accessed from the roof is going to be the best solution, IMO. You will also need to install cables to the battery but that would be done in any installation scheme.
Installing an exterior box to house the controller isn't a monumental task but you have the needed ventilation to contend with. If your ventilation screen or slots are covered in snow or ice, it would be a bad thing. Also, deterioration of the electronics could be possible depending on climate.