Resurrecting an old thread just because ...
The size of copper ID or OD, soft (bendable) and rigid sticks, types M, L etc. explained here: https://en.m.wikipedia.org/wiki/Copper_tubing
The size of the gas line used is based on the number of BTUs used at each fixture/appliance.
So for example my 10,000 BTU water heater using type L soft copper is perfectly happy with 1/4" ID, 3/8" OD for a pipe run as long as about 80 feet. Propane Gas - Pipe Sizing
If I add up all of my propane requirements of hot water heater, fridge, furnace and range top I get roughly 60,000 BTU which is more than covered using 1/2" black pipe or 3/8" ID, 1/2" OD type L soft as my main distribution line from tanks under the belly to the entry point up into the trailer.
The two stage regulator near the tanks drops the tank pressure (~200 psi) to 10 psi then 11 inches water column.
Testing a new system or one that has been opened for repair is done first with compressed air - some claim ~3 psi for 10 minutes (11 inches water column is roughly 0.5 psi). Once there are no obvious leaks, the normal annual checks can be made.
Or I could be confused ....
Signed, a retired stationary engineer turned rv mechanic wannabe