Trucker,
White spirit should be OK, but in amplification of Damian's post - there's more types of plastic made than you can think of - but a caravan plastic panel should be made from either GRP or ABS capped material, and white spirit should be OK.
But it will also remove any wax or surface sealer (if you've used such a product) from the caravan - and you'll need to re-wax and seal afterwards.
Robert