Well, today I finally got around to pulling the fridge out of the trailer. When I got a look at the back, I found that the heater element is a single copper cylinder about 4 1/2 inches long and about 5/8 inch in diameter, made by A.Birch & Co, somewhere in England. It slid easily out of the surrounding tube that attached to the refrigerant tubes. It was not held in by any paste or other sealant, and this seemed ot be the original configuration. It was next to the
propane heater tube, which was also attached to the same refrigerant tubes. There was no significant rust anywhere that I could see. So, question 1 was answered, in that there is only one refrigerant tube and all the heaters heat up the same refrigerant.
The heater element has 4 wires sticking out of it, so both the 110V and 12V lines run the same heater. There are likely two elements in it, but there is no way to separate them out or replace just one.
I took the opportunity to clean the gas nozzle with alcohol and tidy things up a bit, then got to work on the 12V line. The ohms measurement across the 12V wires was 4 ohms, at the point where it connects to the trailer. This, as noted last summer, is not sufficient to get me to 75 watts. I need about 2 ohms to get 75 watts out of a nominal 12V line. So, I opened up the control panel where the 3-way switch and the thermostat reside. I disconnected the line that goes directly to the heater element and measured the resistance. It was 2.2 ohms across the heater element. This looked promising, as a 13 volt battery would give me about 75 watts. It also meant that I was losing 1.8 ohms across the 3 -way switch and thermostat wiring. Since both those components should be essentially 0 ohms resistance, it was probably bad connections.
So, I took all the connections apart and cleaned them. When I put the whole mess back together, I was getting 2.2 to 2.3 ohms across the whole system. This seems pretty close to the required load.
So, I replaced the all the parts and put the fridge back in the trailer. It seems to be working, but I will see how it does for a few hours. I put a glass of water in the fridge and will see if it gets cold. Anyway, I learned two things: if this doesn't work I can't replace just the 12V heater, and dirty connections can make a big difference.
All in all, I learned a lot today and am glad that i tried to fix it, even if it ends up not working.
Rick G in Alberta