Instead of going under the trailer, I would find an wall in the storage area that may adjoin a cabinet and could gain easy access to your electrical center. Of course that depends on your floor plan. Or, if there is a 12V source in that area, tie into it with a separate fused line. If you can get to the electrical center, the ground would be right there also. You wouldn't be penetrating the floor that way and probably wouldn't need as much wiring. A lot of cabinets have openings between sections that could be used to route the wiring to a 12V source.
