Their is no reliable way to get a chassis ground into the skin of the average travel trailer. Car battery to car frame, car frame to 7way connector, 7way connector to trailer frame, trailer frame to aluminum panel, aluminum panel clipped to aluminum panel and small copper wire from the light held by a loosely attached self tapping screw to paper thin aluminum panel. What could go wrong?This is how my WFCO main distribution panel is wired. The negative/ground bus bar is "floating" outside the panel. I say floating because the factory never attached it to a surface, it's just hanging out in the compartment. One of these days I'll secure it.
I don't think any of my running lights use the chassis. It's been a while since I fiddled with them, but clearance lights all had two wires that ran all the way to the panel.