A LPG tank with 120 l only fits 96 l because of safety precaurions. There is an automatic shut off at 80%..
I also do not care about norm weight, it was only to give you an idea about the whats en whens.
I do not believe, that the rear structure of a Flair chassis will hold 1200 kg. „ A friend of a friend …..“? Can you prove it written?
I habe been in business with Niesmann since 1987, I have driven them until 2021 and I think I know a little bit about Flairs.
But- it is your camper, so do what you like.