I would understand the first sentence to mean that the food was actually lying on the floor of the truck, not on plates or in any kind of container.  (A common expression is "her kitchen is so clean you could eat off the floor.")  The second sentence would mean (to me) that they sat on the floor of the truck while they ate.  The food might be on plates or in take-out containers or in a picnic basket or whatever, and the containers would be resting on the floor of the truck.  I would interpret the thrid sentence to mean the same as the first, but it would be somewhat less common.