Couple more trailer ideas:

Another way to waterproof connections is to use a hot melt glue gun. Squirt melted glue into the shrink tubing first, then heat it and as it shrinks the hot melt glue will ooze out of both ends.

After all of that trailer wiring and rewiring is done make sure that you seal every opening in the channel or tubing where you ran the wire or else you will prbably be doing it over again in a year. Why? Mice, they love to crawl into trailer frames and chew wiring all winter. Come spring your lights will do all sorts of weird things when the insulation is gone from the wires.