The Natioinal Fire Protection Code NFPA 1192 and the other codes is references goven the work you would like to undertake in replacing the gas lines. This code (and most codes for that matter) are built on best engineering practices and on the lessons learned from many tragic events.
In installing a new gas line you should follow the code and install new copper piping under the trailer then transition to a hose approved for LPG service to connect to the regulator and to connect the regulator to the tanks. Make sure that you also install an electrical bond between the copper line and the chassis of your trailer.
If you have any propane lines that run inside of walls of your trailer, now would be an exellent time to abandon them and seal them off. If you need them to opperate applinces that you still use, run new lines that are accessible and run through walls and floors only at right angles. A hidden leak could ruin your entire aftenoon!