The solenoid valve should be on the tank, immediately after the pressure regulator. At least, that is where the ones I have seen were, and where the one on my Southwind is.
Shut off the gas valve at the tank, remove the solenoid valve, connect the line to the regulator (you may need to buy an adapter to do that), turn the gas back on, and check for leaks.
Replace the propane detector with one that does not have a shut off valve as soon as you can. Yes, you can get along without an alarm, but remember, it just might save your life!
By the way, if you have a flatulent dog, and he lays by the detector, his flatulence will set off the alarm!
If you have the problem, yours will, too!
