Try using dental floss! The plate will normally be attached with double sided tape and floss passed behind the plate and then used with a sawing action will cut through this without damaging the caravan or the number plate. Using a scraper runs the risk of scratching the 'van or breaking the plate.Hi Mel.
It's probably stuck on with double-sided tape/cushion tape.
This is also the best stuff to stick your new plate on with.
You can pick it up at most car accessory shop.
To remove the old plate try forcing a wallpaper scaper up the back of the plate to cut the tape.Remove what's left with 'Stickystuff Remover' available at Wilkinson's,Woolworth's,etc.
Dave