a water leak into the cabin will introduce enough moisture to fog windows. So will wet floor mats, or wet rain coats, etc. Especially if the truck sits outside, and never gets a chance to thoroughly dry out.
A/C will definitely help with moisture control, as it will condense the moisture out of the cabin air and drip it to the outside ground. If you're having fogging issues, look for water leaks and fix your a/c.