Hi Marc, Thanks for taking the time to generate the drawing. You appear to have a good understanding of 12 volt logic. When I have had to sort out an issue like yours and there are no prints, I do the same as you. First of all, are there 2 different switches on the dash? 1 for "boost" and 1 for battery charging?.
To try and answer your questions, this is difficult because you don't know how many fingers have been in the pie! but here goes: 2 GA could be the alternator or jacks pump, 12 ga could be the monitor panel, the relay is nothing more than a remote switch from the dash switch. Relay logic is this: #30 is 12 volt power source, 85 is from the switch 12volt, 86 is the relay ground, 87A is normally closed and 87 is normally open. When you activate the switch, 87 sends 12volt to whatever you are trying to activate.
I would fabricate a 40' jumper wire with 2 alligator clips. If you want to identify a particular wire, disconnect it at both ends and perform a continuity test with a meter then label it. You will sort this out!!!!
Thanks for the pics of the propane detector. Blessings, Fred