The first mistake has already been made, don't trust them!! Many dealers don't even complete the pdi, and just wait and see what problems if any arise, overall this is cheaper for them. I'm not saying yours would do this, just be aware.
As far as what to check, that's easy absolutely everything. Eyes wide open from the moment you get to the van, check the outside thoroughly for any marks and dents or repairs. I've heard of one dealer local to us that if they dinged a van moving it used to send it for an overnight quick repair and sell it as if nothing had happened.
Make sure that all the systems work, not just that you're shown how they work. For example check that the heating and hot water actually warm up. The van should be connected to water, mains and gas for everything to be checked. Be pedantic it's your money and your van. Do not be rushed through things a hand over for a new van can you say half a day not the dealers' half an hour. You've invested heavily and better to find anything right at the start than on your first trip away.
Have a great time and enjoy your new van.