{I posted something similar in another thread with a different topic but thought it might be interesting as a stand alone thread}
During a recent project (new batteries) I decided to do something about the rats nest of wires that the factory created under my trailer. Where the umbilical cord meets the trailer wiring they used a metal residential junction box rather than one of the purpose built connection boxes that cost less than $10 at most RV or Truck Supply shops ($9.99 on Amazon).
When I started to unravel the mess of wires so I could replace the cheap, non-waterproof, box and make connections in an orderly manner I literally had two large wire nuts fall off when I touched them.
The factory gathers up all the negative wire's from the chassis lighting, the negative wire from the frame, and the negative wire from the 7-wire cord and twists them together in a huge wire-nut (blue I think). It then gets a quick wrap of electrical tape (apparently to keep it from falling off) and that's it.
Ditto for the charge wire from the 7-wire cord, the connection to the battery after the circuit breaker, and a couple others I forget where they were connected. Same large wire nut connecting all these wires, most importantly the one that goes off to charge the battery while driving (at least a little).
Other connections from 7-wire cord to the individual lights and brakes were mad using butt crimp connectors and of the 4 I remember, 2 of them pulled off with very little force.
If you own a travel trailer with one of these connection boxes on the frame it might be a good idea to open it up and if you do nothing else check to make sure the wire nuts are making good connection with the wires they join. Also give a gentle pull on the butt connectors to make sure they were actually crimped to the wires.
I decided to replace the whole mess with a regular trailer connection box so I spent an afternoon terminating every wire in or out of the J-box with appropriately sized ring connectors, crimped properly, and no more residential box for me.
It now looks like this:
Nice secure connections, waterproof, and should I ever need to troubleshoot, all seven basic circuits are arranged in a nice row.