If you're having trouble killing the driver of the dump truck in the junk yard, simply shoot the pilot's cockpit with gunfire. Once the glass breaks, you automatically stop the truck. Do it quickly since the dump truck will automatically kill Jenny on its second pass.
When your about to proceed to El Capacito, have your men go to the statue in the center to take cover. Shoot down the helicopter using a few RPGs. If you run out of RPGs go back to where you had to shoot the APC vehicle to get more.