WHAT tow vehicle and WHAT trailer do you have?
Fridge will run on propane with DC power while in transit
Fridge will run on electric with DC power IF you have AC power (generator/park power or inverter) to operate the AC electric heat element.
DC power is needed for circuit boards/control functions etc.
You have 2 electric systems
a 120V AC System
A 12V DC System
7 pin connector (IF all pins are wired and used) allows for tow vehicle to operate trailer running lights, tail lights, turn/stop lights with tow vehicles if also provides circuit for brake controller in tow vehicle to activate trailer brakes with tow vehicle brakes for stopping AND would have a DC Power circuit to maintain trailer battery charge while tow vehicle is running (from alternator)
This DC Power circuit is just enough to maintain the trailers battery which would provide DC power to trailer.
LP Detector beeping......could be due to trailer battery low voltage, bad/old detector (expiration date 5-7 yrs) or LP Leak detected.
When posting questions..........please provide Brand/Model...type of equipment etc.
Less of a guessing game when complete info is posted.