Don't know it would baffle it so its not a dangerous but weather it would stop the van rocking im not sure.
As there is hell of a lot of water in them so i would think you will get some rocking but not as much as an unbaffled tank.
Myself i wouldn't use an IBC container,
I would get a container that is properly baffled out then you don't need to worry to much.
Gardiners would be able to get you a baffled tank made to the size you require im sure it won't be a lot more money.
As baffling an IBC tank you are going to need a few waste pipe's
http://www.gardinerpolesystems.co.ukPaul