We agree that a water truck as you describe it would fit into the definition of mobile equipment under the CGL form.

There are at least two portions of the definition that encompass the water truck. First, it is a vehicle “designed for use principally off public roads” since it is used to dampen the terrain at construction sites; this description is in paragraph (a) of the mobile equipment definition. The second part of the definition that applies is the part that describes a vehicle “maintained primarily for purposes other than the transportation of persons or cargo” — paragraph (f). The water truck is maintained primarily to help in the construction business of the insured and not to transport people or cargo. That is what this part of the definition is intended to cover; that is, a piece of equipment used in the business of the insured where the main liability exposure is on the construction site, not on a public road.

Note also that the section of the definition dealing with self-propelled vehicles with permanently attached equipment bolsters this construction site exposure viewpoint in that it talks of equipment designed primarily for road maintenance being considered as autos, but equipment designed for construction not being considered as autos. Your insured's water truck is used in the construction process, not for road maintenance, so it should be seen as mobile equipment.