How do I find and repair leak in propane lines?

In my 68 Ambassador, a check was run on the propane system and it took about 2 ½ minutes to lose pressure, which means there is a leak. I was told that it should hold pressure for at least 3 minutes, and preferably 5 minutes.

I see that the lines run under the body of the chassis.

Has anyone else had a lead repaired? Is it costly, a nightmare, etc?

Is this something I can fix myself (not a mechanic), or can an RV shop or specialist fix this easily?

Thanks! Jim