Silage
be careful here as the law changed in May this year ! Basically bowser's were really IBC's ie a tank bolted to the trailer. This meant you didn't have to MOT the trailer every year ! Now all IBC's ( carrying flammable liquid ) have to be twinned skinned, which take out loads of the old ones.
2000 litres is as big as you can go using a normal 4 wheel drive capable of pulling a 3.5 ton trailer otherwise you will need a linked braking system !
As your bowser will be over 1000 litres you will need an ADR licence to pull it