Obviously a baffled tank. Mine is a 500litre from purefreedom.
Are you referring to the wydale tank, ? Unfortunately that big slot in the middle is there for moulding and strength purposes, without it the tank would bulge out of shape and possible split, if your using the flat version of any of these then the "baffle" is useless as water can freely run from side to side or back to front.
My van had 2 of these professionally installed mounted inside a stainless steel frame by pf. - the rocking motion was terrible.
Ibc tanks are marginally thinner walled but have an intigrated frame - the newer tanks also are of a moulded groove design ( not smooth sides ) which just like the bends and angles in a car panel make it much stronger , and filled with irrigation pipe, baffle balls, flower pots you get zero water surge.
All tanks need to be secured properly, I'm sure there are more wydale "baffled" tanks badly fitted than ibc tanks,
The bespoke tank with baffles is the ultimate - low level, baffled and made to fit the van.
Darran