In addition a mandated safety device will not let gas out of the tank if the line to the trailer is not FIRMLY connected, if the thing is just a little loose, no gas flows.
You see, if the fittings are a bit loose (And gas were allowed to flow) you could get a build up of gas near the tanks.
If you get just the right mix of gas and air, and add an ignigition source, Spark, Match, Coffin nail.. What do you supposed happens?
The instructor liked my answer: You land one county over.