A friend of mine has got a campervan. For many years she struggled on without an electric hook-up but she eventually decided that it would be worth investing in a proper hook-up cable. Wisely, she opted for a 3-socket version, because she now seems to use it for everything from a TV to an electric kettle (as well as her coolbox).
The cable she bought wasn't cheap but it includes an earth leakage circuit breaker for maximum safety and I can vouch for the quality. If you're interested, you can buy from the same supplier, here:
http://www.essexcamping.com/code/products.asp? recid=142&subcatid=33
[If you're certain that you only need a single socket, you could save a tenner by buying this version:
http://www.essexcamping.com/code/products.asp? recid=360&subcatid=33
but I'd suggest opting for the flexibility provided by the 3-socket version]
So, by buying a suitable cable, you'll be able to use your coolbox but you'll also have additional flexibility if you later decide you need to plug in other mains appliances.
Chris