possibly a ground issue, but it could be the wire in the plug on the trailer is not on the same pin on the truck plug, there are 2 tail light pins on a 7 way plug. Thats probably why they worked on your sons truck, but not the rv. Turn on just the tail lights on your rv and check which pin is hot, then do the same on your sons truck, I bet they are in different locations. I run a jumper wire to both pins on my trailer so they will work on whatever I decide to pull with.