Civil dispute; property damage

If this was "clearly a result of negligence on his part" as your question indicates, then he is responsible for the damage. If your question leaves out facts that would make it an "act of god", then he may have more limited responsibility. If the storgae lot people did something in the way they parked the two vehicles that may have contributed, they may be responsible, but you must read the storage contract (or have a lawyer do so) to determine what liability or limits of liability are contained in the contract. Need more facts to give a better answer, but if this was clearly the 'result of negligence', the camper owner is responsible.