The original main line was most likely the half inch you mention, and the branch lines were all 3/8. I don't think anything was changed by a previous owner. So, I think your system is pretty much original. Airstream used the same bands all over, for the most part. A lot of gas can be carried through even 3/8 lines with the short distances involved. If you are having problems, the first thing to do is to check the pressure from your regulator. Better yet, just replace it, it is 16 years old now, close to it's general life.
If you have a crimp in the refrigerator line, repair or replace it. More likely, after this time it simply needs the burner and burner tube cleaned. Rust and spider webs plus mud daubers get into things and they can't work the way they were designed.