I briefly had a 400 litre tank in the back of Citroen XM estate.
The hydropneumatic suspension coped perfectly with the weight as did the brakes.
The problem I found, after eliminating all leaks and drips, was that the presence of the water inside the vehicle caused excessive condensation which then made the interior carpeting damp which meant that I could never sufficiently demist the windows.
I then discovered pick up trucks and all my problems were solved.