Idroba is correct. My 67 has soft copper flared connections made at the factory originally and they are still good. 10 years ago, I had a new HW heater installed by Richard's RV who used soft copper flared fittings and that I think is the only way to go. If you are worried about stiffness of the connection putting stress on the appliance, put a loop or dip in the line to provide a little flexibility. The suggestion to move down to 1/4" soft copper couldl be the way to go but I wouldn't go smaller than the connection provided on the appliance. The only flex line on my trailer is from the LP regulator at the tanks to the 1/2" supply line installed down the center line of the trailer to which each 3/8" supply to appliances is attached.