Help! Issue with No Interior or Exterior Lights on Trailer!

I ended up going to a repair shop this morning. Most of the connections were in bad shape as was a fuse coming off of the battery, lots of grime. The real culprit was a ground wire under the trailer leading up into the electrical panel. The screw had rusted off and two large ground wires were dangling. I thought all of the grounds were up by the hitch. Thanks again for the suggestions.