Where should my propane lines be?

Well... the lines should not be exposed in the interior of the trailer. That was an add on by someone.

Basically, here is what the propane system looks like:

The tanks are on the tongue with a regulator connected. (often directly to the tank but can also be connected with a propane hose) From the regulator, another rubber hose goes down and connects to a fitting at the front of the trailer, usually 3/8 flare male.

The gas line runs under the trailer. Just after the main fitting connection, there should be a tee for the gas lamp. 3/8 x 3/8 x 1/4 flare. That fitting is no longer available, so you would reduce from 3/8 flare to 1/4 for the gas line. (the gas line runs up the front of the trailer (inside the wall) and into the upper cabinet. Many times, gas lines are not secured inside cabinets but you can.

If you run a new line, black iron runs from the front to a tee for the stove and heater. Run one line over and up to the heater and the other over and up to the stove. At the stove and heater location, you should use copper up and into each appliance. (if the old copper line is good and in place under the trailer, you can still use it)

You will have to flare any new copper lines according to the fittings on the heater and stove.