The amount you should insure your truck or trailer for is the actual cash value. In other words at the current value, not the value it would cost to buy a brand new truck or trailer.
This is similar on how personal autos are covered by insurance. In the event of a loss the insurance pays on the current value taking into consideration the condition and age of the property.
For more info check out our blog post: What is the value of your food truck or trailer?