I would not run a gas line under the trailer.
I ran the road for twenty years, coast to coast and much of Canada. Trust me, there are plenty of things laying out there on the road that could do some serious damage to an undercarriage. And I would say a good number of times I never saw them coming, so no slowing down or quick maneuvering, you just go over it and hope your fuel line on the truck or air lines on the trailer don’t get ripped off.
Hit a road gator, tire casing, just right at highway speed and watch the sparks fly. You can’t swerve to good when pulling a trailer, seen many attempts with not so good results, and I am not talking about semi trailers. I would run the lines inside the trailer if possible.
Just my .02