Onboard water: over the top?

wallacebob

I have been considering a new (used) van, and some of my choices come with onboard water. I can see the advantages and disadvantages. BUT....

a largish german make van on the site I'm staying at just now -  London, non CC - has a roof mounted plastic tank that is easily 1 cubic metre! I estimate at least 1 tonne of water? I didn't see the van arrive, but would be horrified if it travelled with
a full tank, or maybe worse a half-tank surging side to side, and back and forth. I'm sure Hobby owners will state how strong the roofs are, but really?

Is this normal? Have I led a very sheltered caravan life!