two-wire? three-wire LPG detector connector? what?

Educated guess here - since it is a LP detector, it could have turned off the flow of LP into the rig when it activated. Not all rigs had such a system - so even though your original detector had 3 wires, it might not have functioned that way. Check the LP line from the tank - if you find an in-line electric valve, there's your answer. As long as you have propane flowing, based on your test, i'd think you are all set. ST