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Very strange. I am not sure any of the following will be helpful, but.........
So if “either” the fridge or the thermostat are turned “on” you lose 12vdc to both appliances, but don’t blow any fuses (so not shorted). Sounds like these loads are dragging down the 12vdc feeder source voltage somehow though. It is unlikely both appliances present an abnormal load (failed at the same time) so I wonder how solid the 12vdc feeder wire source is? Do you read any DC voltage across each appliance when either one is on...... or is it zero volts DC on the feeder or some other low voltage value DC?
If you leave the appliances off and put a small 12vdc load (like a 12v fan) across the same 12vdc feeder wire, does the fan run OK or does it also drag down the source voltage (might depend on load current too)? If it also drags the voltage down I am wondering if the 12vdc source is actually somehow abnormally coming through some other load (like a 12vdc light bulb), so as soon as the appliance is turned on more current is drawn through the feeder wire, the voltage splits between the “on” required appliance and the “other load” in series with the normal fused feeder. How would this happen.......not sure.....bad grounds or other wiring issues can cause currents to flow through incorrect paths and other loads sometimes.
Tracing the feeder wire all the way back to the converter fuse panel.......or where ever it splices to get its 12vdc would be best, but probably very difficult. You could try running a temporary new appropriately sized feeder wire from a spare appropriately sized fuse on the converter fuse panel to the appliances. (also temporarily disconnect the original appliance 12vdc feed wire and tape it.)......then try operating the appliances on a known good 12vdc feeder. I don’t want to burn up your coach either so make sure you use proper electrical solutions if making any changes etc. Good luck.
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