Don't want to sound patronising but, you know you're caravan best, but are you sure you have not got a 12v socket near the tv aerial point. All the vans I have had have had one. It is a small two "hole" socket that takes a plug with 2 flat pins. My present one is incorporated in the tv aerial socket cover and embossed 12v, max 5 amp.
To give me the "best of both worlds" I have made up an extension lead with a flat pin plug on one end and a car cigar lighter type socket on the other which enables me to run car accessories, eg, an electric tyre pump from inside the 'van.
If you don't have one of these 12v sockets you could fit one as an alternative to the previously suggested car type 12v socket. The sockets and plugs are freely available from caravan dealers. "Towsure" have both "2 flat pin" and "car" types of socket and plugs on page 129/130 of their current catalogue.
If you do need to fit a socket from scractch I would suggest that you fit an in-line fuse in the cable. My fitted socket is fused at 5 amps max.