I travelled here which was right across the country from London to SW Wales a journey of about 6 hours and with my cat.
My thoughts for what they ae worth would be to buy some large crates like used by rescues and stack them in your van.I assume your van is fairly large??
Also aslo assuming that your cats get on together?
On large crate shou.d take at least two cats plus litter trays.
I think peeps on Purrs recommend pet remedy plug ins and should also come in a spray I would think. so your van could be sprayed with that as a calming thing..................practice with it in the house first and also with the cats in crates.................leave the doors open and put bedding in and toys and let them wander in and out at their leisure.
I will find a link to crates that I mean and add it.
http://www.petplanet.co.uk/product_group.asp?dept_id=773&pg_id=641 needs to have a removable tray at bottom which will catch accidents and also give support to put bedding etc on. They will need to be firmly fixed to your van to stop them moving. Also to get ones big enought to allow cats space to move around.........I am sure a local rescue will take them off your hands should you want them to go after you have settled.
Someone else will be able to tell you about the effects of zycline sp? which I believe will also have a calming effect but is not a sedative which would not be good for them.
It maybe a cats chorus for an hour or so
but suspect they will eventually go to sleep!
Take food and water, spare bedding and large plastic bags for wet bedding etc and stop at service stations regularily to feed give water and change litter trays and bedding if neccesary. Remember to always have the van doors shut when doing anything where the cats could escape!
I was lucky cos I was driven and my cat sat/lay in half her carrier wearing a harness which I could attach a lead and on stops she would take me walking round the service stations................people did double takes when they realized it ws a little cat not a dog
We only stopped when she was awake or restless.
Good luck with your move, you can do it