You dont want to do a repeater.
Off the shelf repeaters are low powered junk first of all, second of all a wireless repeater has to split its bandwidth in half and thus will have very noticable perfomance drop.
You should get one device connected to an antenna on the top of your caravan as a wireless bridge, then via an ethernet cable connect it to a router.
You can use this device which has an antenna built in, you will need to aim it at the source of the wifi. Should be good for about 100ft away.
http://www.amazon.com/Ubiquiti-LOCOM2-US-NanoStation-lo...
If you require a farther distance then you should get a ubiquiti bullet and seperate antenna instead.
You will then need a wireless router for inside of your caravan to provide wifi to your computers/devices.
You will need to use a wifi analayzer program (they have free ones for android, iOS, windows) to find out what channel the park wifi is on so you can set the router to a different "non-interferring" channel (1, 6, or 11).