The LP pipes are always (on all units I have seen under) on the exterior
of any RV to points as close as possible to the appliance. Then the pipe
goes up through the belly skin and floor to the appliance.
LP (propane and butane) is not like natural gas (methane), which is
lighter than air and rises quickly if it leaks. LP is heavier than air
and will "puddle" in any low spots.
My travel trailer has black iron LP pipe underneath, then copper up to
the appliances.
Al