I think this may answer the original question, and also the spluttering when filling up, at least in terms of what happens on our van.
I don't know whether this is standard on a Hymer, or an after fit item, but it's just a tap like used in beer making or some such other pastime.
When filling up, it's manually opened to expel air, and when it starts chucking water out, the tank is full.
When tank full, just close the tap.
If tap isn't closed after filling, we can be tracked by the following the stream of water on the road for a good few miles
There is a one way valve on the overflow, to stop air being sucked in, and there is also a breather pipe at the top of the tank, with a one way valve, where air can be drawn in, but not expelled.
Both these pipes come from a t-piece fitting on top of the tank.