I had the same problem with a propane odor I could not isolate with soapy water. Finally pulled that red regulator valve and found it had a small vent hole under the valve with some oily residue around it. Was not getting soapy water to it to bubble. Replaced that valve and no more odor.
Now as to why that regulator valve is there, here is why.
https://www.montanaowners.com/forums...ight=regulator