Probably is 3/8 copper and will need to rerun through that plastic hole to a junction under the trailer. I think I used wrong term flange instead of flared. The flaring tool creates the lip, make sure the nuts are on the pipe before flaring.
I got all the tools and hardware at ACe hardware.
The challenge is only the delicate copper. I needed 10 feet, bend 90 up at hole in floor, loop up and bring down to back of water heater, bend at heater's hole, flare and seat. Destroyed quite a bit of copper at trailer hole and water heater hole bends.