Run wire under trailer, near the frame. If the under carriage is covered, run a line exterior, just affix real tight so it won't dangle for flop in the wind. Go under the camper, not through the camper. If you "can" remove the under carriage cover (whatever your's is, if you have any), then it would be more advantageous to run it along your trailer frame and out of sight. I wouldn't go inside the camper.
Your quoted price made me say "OMG!" Maybe because I'd probably do it myself. My philosophy, since I was about 15 years old is, "If someone else can do it, I can learn it and do it too!"
