Some LED's are polarity sensitive. Have you tried reversing the wires on the offending lights? Also check that you have a very good ground between trailer and fifth wheel hitch and tow vehicle. Try putting a booster cable from the 5er tongue to the tow vehicle battery negative as a test to see if ground is adequate.

Just some thoughts as I also had similar trouble converting our 5th's tail lights and it turned out to be a weak ground circuit. When a ground circuit is weak power will always try to find the path of least resistance and that sometimes can be through another circuit causing other LED's to illuminate.

Hope this helps some

Rick.