Gas lines don't tend to come apart in the middle, they are all threaded together. You would need to undo all the metal gas lines from the tanks to that valve, get two unions and what's called a " close nipple" and then reassemble it all back to the tanks. When all that's done you should have the lines pressure tested to make sure you haven't made any mistakes that will burn your RV to the ground.
Honestly - the easiest thing to do is replace the propane detector with a new one that will open the valve.