NO! They would simply equalize. You'd be better off to close the onboard tank & operate off of the 15lb bottle through the extend-a-stay system. Propane is pumped as liquid into the tanks/bottles & operate equipment off of the vapors from that liquid & just connecting the 2 won't transfer the liquid just equalize the vapor pressure.
Danny & Linda
Sold our '13 Thor Redwood 36 FB
Currently RV & truckless