...I figured if a big propane tank can sit out in the hot sun without a problem then the little tanks in the camper would be safe...
If that big tank ever released vapour to relieve pressure, would you know it? It's outside, and the propane would be dispersed, likely without igniting. Inside, it's not the same situation.
But yes, I have left 1-lb cylinders in my Boler
. That was an error, and those cylinders are considered more acceptable inside than the bulk tanks because the magnitude of the risk due to the error is smaller.