The more modern trailers like your '08 Lance are usually equipped with a good set of wires and lamp fixtures. Also there probably isn't a lot of oxidation.
Here's how I would troubleshoot the issues: Determine that the truck's harness has power at all the terminals and that the harness has continuity to ground. Here is a page of the different trailer electrical schedules----
Harness schematics and wire color conventions. Next determine that the trailer's harness has all of the wires connected and in the right locations on the umbilical plug. PLug in trailer harness and observe for power at any of the unlit light fixtures. If you have power, there is a ground issue. If no power at the fixture, there is an open in the circuit. I'd venture a guess that 9 times out of ten, marker light issues are related to a faulty ground. Check real good for the ground wiring to the front fixtures and the first going back along the driver's side. That is where the chain wiring usually begins.
Good luck with the lights. Lance makes a fairly stout trailer so it is probably a minor issue.
