All I do is use a short hose with a piece of copper tube bent into a hook shape that I hang in the filler hole whilst I fill up.
A fully automatic system would need a torbeck (sp) installing in the tank which I would not trust, although Hobby supply such a system called "City Wasser".
My tank is 70 litres capacity so lasts a lot longer per fill.
I believe that the UK tanks of circa 40 litres are too small.
Some caravans come with a 24 litre Fiamma roller tank which is next to useless, one of my recent caravans had one of these, I ripped it out and fitted a 70 litre Fiamma tank.