Safe Transport/Storage of Aux LP Tanks?

I've posted this before but it's a good warning: Years ago, I was transporting freshly filled 20 lb cylinders in the back of a conventional 1/2 ton van. The kid at the pump overfilled them. I was driving on a stretch of two-lane, straight roadway. When the pressure relief valve opened, within milliseconds, I could not see my hands on the wheel. Luckily, it was Summer and I had the driver's side window open. I glided onto the shoulder and exited the vehicle. Had there been a spark or source of ignition, you would not have read this post.

Later, at our camping spot, another cylinder did the same thing. We then, evacuated some gas off each cylinder. Be careful out there!