How about introducing the necessity of maintaining vehicles? It seems quite logical to me that a patched vehicle needs to be maintained quite often. Lets say a component of the vehicle breaks every 12 hours or randomly every 12-24 hours. Furthermore vehicles that are not maintained to full functionality for like 48 hours may despawn. This would cause people to think twice whether they keep more vehicles at their site than actually needed. This is just one approach to solving the problem. Use it as food for your thoughts :)