Thoughts:
1 - Most RV ducting is put together badly, so you end up heating your cargo bays more than you heat your coach. This is good for keeping your tanks from freezing but bad for LP efficiency. Walk around in the middle of the night barefoot and see if the floor is toasty. If it is, then you need to get inside the bays and check as much of the ducting as you can get to and find any fittings that are disconnected or even holes in the duct.
2 - 65° is pretty high if you want to conserve your LP and don't want to run your batteries down. Try something closer to 55-58° and throw on some extra blankets.
3 - We use portable buddy heaters in our bedrooms to keep those rooms warm without warming the whole coach. If you keep the doors open, you shouldn't have to worry about C0 since there's so many cubic feet of oxygen in the coach.
4 - Walk around your coach and feel the window sills etc. and feel for leaks to make sure you don't have heat escaping from a window that doesn't shut completely. Our toy hauler has a small crack where the ramp seals against the frame and it can get cold back there since it's so far from the thermostat and it's at the far end of the ducting.
I would be surprised if it's a propane leak, because that much LP would go kaboom if it were pooling someplace inside. My guess would be that it's just running too much and never shutting off.