Didn't know it'd be this much fun eh?
Correctly installed propane lines are outside as they appear in your questioned image. One must always keep in mind that transitioning from year to year, it seems that Airstream, justifiably so, used up whatever stock was laying around in inventory bins and shelves, so it is very possible to have “last years” design parts.
What year the switch occurred from black pipe to all flexible/soft copper propane plumbing, Who knows? How each different shift of assemblers configured the lines may be a matter of how far they had to walk to get the parts. There's a hardware store a few minutes walk from the jackson center assembly barn. My guess is that you are looking at the original install.
It appears that you have a blend of copper and iron. I'd switch out to all new copper as I feel that fifty year-old iron pipe, especially at the transition nipples ( there is a right and wrong way to do this), cannot be trusted.
It's OK to single flare copper for propane, properly regulated, there's less than one PSI in those lines. Considering the risk, new copper's pretty cheap, and flaring is an easily learned skill.
Inhaling raw propane gas will kill you before an explosive concentration accumulates.