On the Allegro Bay model it removes via the exterior wall panel:
1. remove the exterior access door panel
2. remove the screws and caulking of the door frame "escutcheon". Remove the frame.
3.The body panel on the exterior is held via screws underneath. Panel swings vertical "after" you remove the escutcheon "flange".
4. LP connection is at the front left, power is rear, right-hand.
FWIW: I replaced mine with the same size as my oem unit; but could have installed a larger capacity model in the same cavity had I known it before ordering. I shopped for best deal online and easily saved bucks over Camping World price. If you damage the oem escutcheon frame, you can get another, but it will require paint to match your bodywork.
On edit: Just what "happened" to kill the heater? Are you sure it cannot be repaired?