How canI get compensation if an unoccupied car slid into mine in bad weather?

If the insurance company won't pay, you could sue the owner of the vehicle for negligence.  Negligence is based on the failure to exercise due care (that degree of care that a reasonable person would have exercised under the same or similar circumstances) to prevent foreseeable harm.  A car does not slide by itself; so, either the transmission wasn't in park and/ or the parking brake wasn't set and/or the front wheels not being turned in the proper direction.  In order to prove negligence, you will need to prove breach of duty, actual cause and proximate cause.  Here, the driver breached the duty of care by not parking properly to prevent the car from sliding down the slope.  Actual cause means but for the car not being parked properly, would your camper have been damaged?  If the answer is no, which appears to be the case, you have established actual cause.  Proximate cause means are there any unforeseeable intervening events which would relieve the other vehicle's owner of liability?  If the answer is no, you have established proximate cause.  Your damages would be the cost of repairs to your vehicle.  You will need to mitigate damages by selecting a repair shop whose charges are comparable to other shops in your area; otherwise your damages will be released accordingly. 

You probably can't make them pay. If the other driver placed his car in park and applied the parking brake, he did do everything right, from a legal point of view. He was not "driving" at the time, so the no travel advisory doesn't show that he was being careless or negligent. The standard is what would a reasonable man (or woman) do; and a reasonable person would conclude that parking, even when it's icy out, is perfectly reasonable as long as you put the vehicle in park and set the brake. Consider this: if you could not park on a slope on an icy day, whenever it snowed or sleeted, everyone who lives on a hill or has a sloped driveway would have to move their car, which clearly is NOT what is done or expected to be done.

You may disagree; and you can try to sue for compensation--but you have to accept the fact that there is a good chance you won't be able to prove that the person was careless or at fault. (Or if you don't believe the factual account--that is, you think the parking brake wasn't actually on--you might sue on that basis.)