In the U.S., most areas don't permit long runs of copper tubing for the delivery of L.P. or natural gas. Instead, black pipe (similar to galvanized pipe) is used, with a thread sealant specifically for gas. You'd probably use 1" or 3/4" for the main run.

Since you are essentially always at home (you don't leave for months at a time) theft is less of a problem, so running the pipe down the outside is probably okay. I know that unattended buildings in Mexico are oft targeted for quick pipe removal by n'er do-wells. You remember Geoff Orth from the Tuesday Night rides -- his place has had stuff removed twice.

Have you tried the soapy water test? Get a very soapy solution of dishsoap in a cup, use a paint brush to "paint" it on fittings and even where the pipe comes out of the wall. You may or may not find a spot where it blows bubbles. Test everything you can find.

25 liters a day. Wow.