My 1979 Boler
B1700 - which I believe has the factory original propane plumbing - uses iron pipe with threaded fittings
under the floor (open to the road), and copper tubing
from tees in the pipe at floor level to each appliance. This makes sense to me: the iron pipe is strong so it can be properly supported in a horizontal position, and resistant to damage by road debris; the copper tubing can be bent to reach the appliance without extra fittings, and is protected in the interior.
As Francesca explained, this is smooth plain copper tubing
(presumably of appropriate grade as Floyd mentioned, of course) - not the corrugated stainless steel tubing which is used in houses, particularly as the final connection to an appliance.
I believe that flare fittings
(the tubing is flared to fit over the tapered end of the fitting, and clamped on to it by a flare nut) are the standard practice for propane connections. Certainly every propane appliance inlet, plus hose fittings intended to be removable, on my three RVs are all flares, not compression fittings.
A nominal tubing, piping, and hose size
of 3/8" seems to be normal practice for all parts downstream of the regulator (and 1/4" upstream).
Originally Posted by stevebaz
T the last junction of your maine line. Use a Tee with a cap on it. That way if you have to flush the line you can remove the cap and disconnect at the regulator and flush the whole line. You can also disconnect at the appliance and remove the end cap and flush out.
This makes sense to me. A flare rather than pipe thread fitting for this end-of-line flush port would be ideal (even if it is on threaded iron pipe), because a flare cap can be more easily and reliably removed and replaced without subsequent leakage than a pipe thread cap.
I had a furnace
die in a trailer, and the repair tech attributed it to liquid contaminants accumulating in the piping and reaching the furnace
. A dead-end vertical section of pipe can be included a low point in the iron piping to act as a liquid trap, but that takes vertical space which we don't really have on most of our eggs (my big trailer has this, but there is room for it).