If you are talking about the propane lines they must be located outside, below the belly pan as they were originally. A leak in them, if they were in the belly pan, would cause a gas build up and potential explosion. Propane is heavy, and will pool inside the belly pan.
The system used is flared soft copper pipe, as original. All connections other than the last one to the appliance are to be made outside the trailer (below the belly pan). Airstream did it that way in 1966
, and still does it in 2016.
Flex pipe of the type used in homes for final connections to the appliances and available at Home Depot etc. is not legal in RV use and not approved for such use. It is only for occasional flexing, not the continuous type that would be encountered in an RV.