Thanks for the suggestion Pahaska! I took your advice and bought a monometer from the local propane and propane accessories store, Smith and Smith Propane in Killeen. Vintage Trailer Supply has the same low pressure test kit on sale for $30
. I paid a little more than twice that, but I needed it right now.
I referenced the manual for old Dometic fridges and hooked up the manometer to the test port. With the temp selector on "MAX" it read 4" not the required 11". Problem found. Now to fix it.
I tried to adjust the thermostat "Cold Adjustment" screw. It didn't change the pressure at all. I figured that I must have some gunk in the propane line somewhere. Sure enough I found a big old plug of unknown substance inside the line to the fridge. I cleaned it all out and re installed it looking for the magic 11". I was disappointed to see 4" again. This time I took the gas thermostat off completely and checked the pressure at the gas valve for the fridge. 4" there as well.
I then turned my attention to the pressure regulator at the tanks. I disconnected the outlet line from the regulator and checked it there. It was reading 10". Since I read in the service manual how to adjust the regulator I adjusted it up to 11". I know I know, all you doomsdayers are thinking I'm going to blow myself up about now, but I watch McGuyver when I was a kid so it's cool.
I narrowed it down to line blockage between the regulator and the Tee for the fridge supply line. I took the line off and flushed it with alcohol and blew it out with compressed air. Some more chunks of I don't know what came out. I reinstalled all the propane lines and did a leak check with soapy water. I hooked up the manometer to the test port and it was at 11".
The only major problem is that while I was trying to adjust the thermostat with a blocked propane line I unscrewed the adjustment screw too much and the spring came undone. This caused the outer stamped portion of the thermostat to fall off from the other portion of the thermostat body. I now need a new thermostat for real! Luckily Dometic has them on the shelf for $120.
Since the thermostat is toast, I took the little spring and valve from the inside of the thermostat and now it is on "Max" all the time. It was 100 degrees when I closed the door to the fridge. We'll see what the thermometer says in the morning.