Mine is plumbed mostly with the rubber coated flexible propane tubing. It was inspected by the State of Oregon so it must be OK at least in OR. On our frig and cooktop the flex hose is connected directly to the appliance since the hose has a flare conneciton on it.
The Propex requires a copper tubing final connection to adapt to it's foreign threads. IIRC the adapter is a compression type and is provided with the furnace.
You can just see the adapter on the furnace.
Copper tubing down through floor, along with inlet and outlet air tubing.
Adapter from flex hose (flare) to NPT to copper compression under the van. I put the adapters under the van because space was tight near the furnace, and to minimize propane connections inside the van. I was also concerned that the hose might put too much stress on the furnace gas connection.