We had an Autosleeper Clubman GL on a T4 but no, didn’t get a tank fitted underneath. We had 2 gas bottles in the cupboard, never had a problem with that set-up.
Our current AS Nuevo does have one. It’s getting increasingly difficult to find LPG (or Autogas) and the tank has to be replaced every x years too, it’s not “forever”, so it’s maybe a feature with a relatively limited life.
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.