Go "shopping" at an RV dealer. Look under the new units to see how they did propane. Do what they did. My 2017 Airstream has rubber hose to black pipe to copper when it T's off to the appliances.
My supposition is that propane must be run outside in case of leaks. (My Airstream has an enclosed belly and everything BUT propane is inside. The propane is outside.) Copper is too soft to run outside because rocks and crud being thrown up can puncture it, so they use black pipe. Then they switch to copper when T'ing up into the trailer for flexibility. The copper run for the 2 inches to get up into the trailer is short enough to not be a significant risk.