Using Mark's RV wiring page
, use your tester and assure the all truck plug pins are hot when appropriate. Then check your ground pin for continunity and resistance.
If the truck end is working properly, your trailer wiring has to be the problem. Check the trailer wiring at the junction box inside the trailer front, then trace all the way to the bulb sockets. Include the trailer ground wire as you did the truck's.
Oh, did I mention I hate wiring problems! I've got to find the break in a 12V wire to some interior lights this week.