Well, it sounds like you got several wins, but a couple of bummer losses, and I'm not referring to the voltage profile across that circuit. Mine also split a short distance from the fuse panel. It was my guess that one of the two found its way to the propane alarm and maybe the radio, as you have surmised. There is one more load, I think, and that is the control voltage for the slide switches. That would include the two slide switches on the end of the cupboard over the sofa and the one switch that you probably have in your bedroom. Those are extremely low current loads, even while operating them, as I"m pretty sure they just pick up the relay coils that then switch the slide systems, themselves.
I did as you did and opened up the area behind the propane detector, but did not check for the voltage drop. That was last fall, before I had learned as much as I have since then. I was just looking for wire colors to try to establish the routing of the circuit.
Neither the propane detector nor the radio should result in much drop in voltage. Those two loads, as well as the slide switch circuits, may likely be far more tolerant of a low voltage situation and still function adequately. Also, I wasn't interested in trying to tear into the dash to the point of being able to identify the wire feeding the radio, and whether the circuit stopped there, or continued on the the slide switches and/or the refrigerator.
It is incredibly interesting that you are finding the same magnitude of problem. As was said in Alice in Wonderland (I think), it just gets curiouser and curiouser.
I'm guessing you did not find any kind of east-west raceway between the bed compartment and the compartment under the shower or under any of the cabinetry aft of the refrigerator. Dang! I've not seen the details of the layout of your coach's walls, ceiling and appurtenances to hazard a guess of how you might chase something across and down to the bed.
Note, on ours there is a bundle that exits the floor of that compartment under the bed. I used that fact to run come cables to upgrade our tank monitoring system. It worked well. You may well be able to safely run a cable down thru that insulation blob into the area below the floor and secure it as you route it across the underside of the coach to where you can penetrate back up into somewhere along that passenger side area you've been exploring. I think that if you choose to run a replacement cable, and if it is I think you are describing it to be, I would give that serious thought.
BY THE WAY - If you are interested in a huge improvement to the tank monitoring system in your coach, let me know and I'll share what we did. EXTREMELY HAPPY! All three holding tanks AND the propane tank on a digital readout. TechEdge iSeries. https://tankedge.com
Keep us posted, and if you want to share pictures or bounce ideas, please do. GOOD LUCK!!!!!!!