If you put a gas line inside a trailer it’s just another place to collect grease and grime, and an area that is hard to reach and clean. There is no area that is more difficult to clean than the area behind a conduit or gas line. Those are the areas that the health departments look for to spank you when you get inspected

And if under your truck is safe enough for your fuel line  or air lines on the trailer, it’s safe enough for a black iron gas line, which is much stronger than a fuel line, or a trailer air hose.

One other thought how many times a year do you hear of someone getting a muffler ripped off their car? On occasion it may happen but not often.

I ran a black iron  propane line under my trailer and have never had a problem, however becuase I ran a 16 foot straight run from one corner to the opposite corner, I wielded a 2 inch angle iron runner under the trailer in 3 foot sections, so that most but not all of the gas line is virtually covered. I did this not for protection but for support. It was easier to tack the angle in 4 or 5 places than do a bunch of brackets to attach the gas line.