What is this?

Simple - it's a ball valve that screws into the side cap of your Aquaroll.  Connect your (blue-food grade) hose to it and the other end to a tap when on a serviced pitch.  Leave the tap open and as you draw water off via the aquaroll pump - the aquaroll will fill up again.  Ball valve automatically cuts water flow off when the desired level in the aquaroll is reached.  I am assuming that you don't have an inboard tank in your camper and rely on an aquaroll for your water supply?

If you don’t get any joy here, there’s sure to be someone on the AutoSleepers forum who will know as they seem to know the ins and outs of every AS model made.

https://www.autosleeper-ownersforum.com

Ah, I see you’re already there, AV, and have been given the same answer as R&R above. Job done 👍🏻

It's a "ball cock" and works just like the one in your loo 

when was the last time you saw a ball cock in a toilet cistern?

When they had had a copper ball 

A while back then ..... 🤣