An auto insurance policy for the fuel truck will cover the liability associated with the operation of the truck.
Spills of the fuel being carried can create a major liability for the operator. Some insurance carriers will cover minor spills by an endorsement to the policy. If the spill is excessive, it is very likely not going to be covered under the auto policy.
To cover the risk caused by spills from the fuel being carried a broad form pollution liability insurance policy should be purchased. Broad form pollution liability insurance will cover the cost for damages caused by the spillage of the fuel, to include the clean up. Hazard material clean ups can be very costly.