Girl, that furnace looks like a rusty hulk. I think you made a wise decision to remove it. I had a '71 Caravel
18 many years ago, and its furnace's heat exchanger had numerous rusted pinholes in it, which allowed combustion byproducts to get into the room air. Big CO (carbon monoxide) risk!
The propane plumbing on our '71 was all exposed beneath the belly pan. And poorly done, with many kinks, i might add. You should be able to see the copper tubing coming through the pan below the area where the heater was, and you can probably find all the tees down there for the furnace, galley stove and water heater. it would be safer to remove the piece that feeds the furnace rather than leaving it and capping it inside the trailer. As mentioned, removing the flare fitting at the tee and replacing it with a cap should be pretty straightforward. A flare cap would be easy to find at a hardware store. Be sure to get a copper "flare washer" to put inside the cap. That will give you a good seal between the cap and the tee. Using a brass cap on a brass tee without the softer copper washer could result in a leak. Good luck!