I've been planning to use grey PVC when I redo my lowboy. hadn't decided if I was going to bother with j boxes.
my issues have always been around chafing at transitions or hangers. I was thinking deburred plastic conduit would have a decent chance of not screwing up insulation.
the more I think about it, the more I don't want to run ANY conduits. Dirt collecting inside conduits can cause problems (unless somehow perfectly sealed) and make any repairs more difficult.
I am thinking to run wires (preferably jacketed) and use Delphi Packard weather pack connectors at all components (brakes, lights, etc.) all hanging wires (at brakes, lights, etc.) will be sleeved with either wire looms or braided sleeves for additional protection. I'm thinking about wires running down the trailer will be either secured with loop clamps every 12"-18" or weld in a long 1/2" round stock along the trailer on one side and zip tie all wiring to it.