Most of the propane
systems in smaller "i.e. FGRV" trailers use 3/8" flare fittings. If that's what you have you need a flare nut for the copper tubing end and an in-line 3/8" to 3/8" flare fitting. You can find the rubber hoses pre-made in many lengths at RV and outdoor places that cater to the BBQ grill crowd. The hoses have a 3/8" flare fitting on both ends. I think the one I bought at Camping World was about3' long and cost about $10. A common brand for the hose is Mr. Heater.
If you don't have a flaring tool you'll have to get one. They cost about $15 to $20 but will last forever.
Don't forget to check for leaks
after you complete the job. Also, do not use plumber's tape on the connections. Flare fittings seal by compressing the flared end of the cu tubing between the flare nut and the end of the flare fitting.
Again - don't forget to check for leaks!