No Ron. These balloons will not do their intended job.
Please note that while driving, the balloons trail behind the truck. Their drag and air resistance is more than their lift.
At extremely low speeds, it might be possible to obtain some minute immeasurable gain.
You could always fill your tires with helium to lighten the load. However, helium atoms are small enough to escape thru the pores of rubber. You would constantly have flat tires. I don't know how th ecost of helium would compare to the gas savings.
If you want to increase gas mileage on a truck, get a tonneau cover. Keep it regularly tuned up. Talk to an exhaust system specialist - some mufflers increase MPG by 2 or 3.
Or, simply stop making non-essential trips.