It does take a while to get used to the controls on foot, and the series has never been popular for its targeting system, but trust me, you get used to it. Try running back away from the laundry truck, turning around, then running straight towards it. When you get to grenade distance, launch it, and it should go straight for the truck. That's how I end up making good explosions if the camera is acting up or if Tommy or the tanner is going apeshit for some odd reason.
Keep with the missions, they keep getting funner and funnier as the game progresses.
-Lasereth