I have never seen a propane tank that was filled properly (to 80%) vent due to a normal temperature increase.
Does anyone know what pressure is required to cause venting? With that info one could use the Ideal Gas Law to calculate the temperature at which venting would occur.
From my design engineering days for the automotive environment I know that under dash temps can reach over 200F in direct sun in the AZ desert. (black dash, black car)
CO law allows propane tanks under 40 pound capacity to be transported in a car or car trunk when secured in an upright position. I would expect a closed bay compartment would be the same.
That being said, a defective valve that leaks would not be a good thing in an enclosed area.