All of the gas lines should be under the trailer attached to the bottom of the belly pan. The feeds to each of the gas appliances will Tee off of the main line and run straight up into the cabin of the trailer and attach to the appliances. My 75 TradeWind had shut off valves at the Tee for the water heater and the furnace. Having the lines running on the outside under the trailer was to prevent filling the trailer with propane if there was a line leak.
You should be able to trace the propane line from the tanks and regulator to the propane line under the trailer. Everything in the pictures looks like water lines to me
Wally Byam Airstream Club 7513