While reading up on Alde Heating system can across this info. As far as I can remember it was in relation to motor homes but I suspect system should be similar. Might give you an idea what to look for.
The anti freeze drain on our Lunar Clubman is in the middle of the caravan right at the back. There is a tube dropping with a brass bung held in place with a spring clip.
Amazing family weekend with old steam engines, classic car displays, market stalls, and full catering and bar. And camping on site - Save £25 by booking in advance.