And a spare corkscrew (or make sure it's a decent one) - ours broke on new years eve and we had to go begging at the caravan on the next pitch to us. Good job we were at a campsite for a change and not on our own somewhere, though I'm sure we would have improvised something.
As above, all used on our first trip: handbooks (why is that light flashing???), fuses (sat-nav's blown all the 12v sockets!!!), velcro &/or bungees (hinges have gone on the kitchen overhead cupboard - won't stay shut when we're travelling!).
Bottles for drinking water (rather than using it from the van tank)
Electric hook-up cable & means of storage
Blue water hose
PHONE & essential numbers eg insurances