Not only illegal but very dangerous. Read all the posts. Probably thousands of people carry them all the time in enclosed compartments.
Just a passing thought. How long can you do it without it having a problem? I would mount a bracket on the back bumper if it was that important to carry the extra propane tank. I saw one in the back of a pickup truck fall over because it wasn't tied down. Valve broke off. After seeing that happen I strapped my extra tank down on its side in the back of my pickup when I had a 5ver. The experience of seeing one propane accident will make an impression that will last a lifetime. JMHO.