Pipe sizing link3/4" !!!! 3/8" is more than likely all that is needed, or even 1/4" depending on the BTU requirements and the distances. (MBH = 1000 BTU per Hour) Less weight to drag around.
Personally I used copper, type L with flared fittings. Type K can not be used with flared fittings, too hard too thick a wall. We ran a class C RV for a couple decades. It had type L copper from the factory. Ran it over all sorts of gravel and back roads and never saw any crush damage from rocks. Not saying copper with take a direct crush between a rock and a hard place (steel frame), but that was my experience. If you use steel pipe do use the black variety mentioned or the special green coated stuff that can be buried.
If you do use threaded pipe use pipe dope /paste, NOT teflon tape. Depending on the care in installation bits of tape can end up inside the pipe and plug small gas orifices.