You would need to take some care to get the whole thing (or at least the whole surface of the thing) up to temperature. For instance, if you did try your truck method, I'm guessing the side that was down against the truck bed would be sufficiently insulated as to not heat up much at all. You would need to put the mattress on a grate of some kind and propped up a couple of inches so it had good airflow.

And of course without far better controls, this method would never be certified, but it's useful to hear that the necessary temperatures are so (relatively) low.