I have never seen this before. It looks like a chunk of fine sand or clay in the propane
line at the tank connection. This would be on my 1984 Bigfoot
with dual tanks. It looks like both tanks feed the regulator ( no switch between tanks that I can find).
I am working my way slowly through this new to us f/g and this is part of the process...closely examining all gas connections, appliances etc. I will pipe goop all joints, check w soapy water etc if I start replacing things but for now just wondering if this blockage in my line looks like this...the other tank connection looks normal.