I've got some spare pieces of that line as I just replaced the lines and fittings on my 416 shocks. I'll check it for the DOT label.That black line should be DOT rated Nylon and will go well into the hundreds (should be marked on the line actually) - the O-ring seals are rated that ot higher.
All the same, I would recommend contacting your vendor or the manufaturer to ensure that the'res no other reason for the low pressure.
Trailers are typically set up with camber to improve tracking, so that contact area seems "most likely", but this coudl indicate a little too much camber or possibly loose bearings or something along those lines. (I'm certain that you've already checked that all else is nominal)