Yes, they can deny you coverage for this reason. A commercial car/truck policy is different, and more expensive, than one for just personal use, since commercial use increases the risk of damage (hauling materials; driving more; etc.). The risk profile and cost of a commercial policy is greater than that of a policy for purely personal use; if you don't disclose (and pay for) commercial use, you have only a personal-use policy which does NOT cover commericial use of the vehicle. You failed to disclose material (important) information about how you use the truck and did not buy the correct policy; they can therefore deny coverage.