Service Areas

I've been caravanning for about three years now, but still feel like a newbie!

We are off on a slightly longer than normal trip this summer and I'm looking at stopping at the motorway services on the way, but one thought occurred to me - caravan security in the car park. I'm paranoid about it being pinched while we pop in for a cuppa - what does everyone do to make sure their vans stay with their car?!

Thanks