We use a cheap pop up tent from Decathlon. The dog goes in it when we're in the UK, we use it to stash clobber (child seats, etc) when abroad. We've never been charged any extra for it and it slots in easily in the boot.
Do think long and hard though SG - it would be a real shame if you converted your van then realised it wasn't possible to use it properly. I know of several who have shelled out for an all singing, all dancing Cali or converted van then realised it's not for them - you won't have a loo, or shower and while you can wash up in the little sinks, it's FAR easier to use site washing up facilities.
Your first night WILL be hard too - get through that (get the kids utterly knackered - ignore normal bed times) and you should have a ball.