I can't see enough of what's going on in that pic. The line on the left appears to lead from the tank valve to a 2 stage regulator. But from the regulator?
That fitting looks to be just below another tank valve?
If it's not, and its in the line coming from the rt. side of the regulator, it's on the low pressure side of the regulator. Fitting like that might be used to supply a grill, or have a line connected to it from a propane bottle using it's own regulator.
Either way, brass is commonly used here. Actually, the tank valves are brass under the paint. No cause for concern...
1997 37' HR Endeavor, 275hp Cat, Freightliner
03 CR-V Blue Ox, Ready Brake