You can use the fridge right up until the last minute, but after you have moved it you should leave it switched off with the door slightly ajar(to prevent bacteria smells)for a few hours or overnight to let the gasses settle again.
...although if it is a fridge freezer you'll have to let the freezer bit defrost unless you want a wet mess in the back of the van! That will take at least a day. You can safely move it either upright or on its side (not on it back as you will damage the cooling vanes etc), then as said, let it stand for an hour or two before switching on.