You probably have Tack Oil on your truck. I work Highway Construction and it is used to bind two layers of asphalt together. I just got done cleaning my work truck today, it was completly covered in it. I use engine degreaser or Citrisol, it is an industrial type of citris cleaner. Some of the other commercail types of citris cleaner will work also. Kerosine works as well as diesel fuel. The tack oil is petroleum based and usually emulsified with water, so all you need is something to breakdown that bond, which a number of things mentioned above will do and not hurt the clear coat finish on your truck. No worries, just use a cotton cloth so it will not scratch the finish,
Good luck.