I assume you have already signed and Order Form, paid a deposit and may/or may not be handing in a trade in van.
I would suggest you treat it as you would any other purchase.
Is the van the model you ordered? Look at the outside carfeully, are there any markes,dents blemishes. The dealer will show you everything inside and out. Obviously he cannot operate all the gas and electrical equipment so that part you will have to accept as satisfactory. However, most dealers will allow you to stay at their expense, on a local site for you to check these things out. Any faults can then be easily remedied, or a programme agreed. Look inside for obvious signs of damge, hopefully there are none. If you have ordered extras, then check they are present and to the specification you agreed at time of purchase.
Once the van is connected to your vehicle the lights can be checked. The dealer will show you how to connect any stabiliser. He will also confirm the wheels have been torqued correctly. In my case the dealer did this infront of me. He will ask you to sign acceptance of the van if you are happy at this point. He will obviosuly expoect you to pay him the full amount due, either by debit card or cash, if you wish to take the van with you. This obviously doesn't affect your rights under the law. If you find minor problems, such as sticking blinds, then telephone the dealer and disuss a course of action. He may say bring the van back, or may agree with you to replace the offending item at a convenient time. If you find major problems on the first day or so then take the van back immediately to see what can be done. If it is a fitting such as a heater the it can probably be replaced without difficulty. If it is major like damp damage (unlikely on a new van) then you have to discuss with the dealer what his action is going to be. If you cannot agree, then I am afraid it is probably time to contact your trading standards office and your lawyer.
There has to be a certain degree of trust between you and the dealer, and most delaers are quite human. If you cannot accept that then find another dealer, or don't buy a caravan.
Good luck