There is no hose-on, cure-all product. You will have to at the very least get the truck up on jackstands, get under there and clean the underside of the truck. With a truck you are lucky in that you can take the body off the frame and have good access to nicely clean up the frame and can turn over the body sections to get at them. Sorry that's the only real way to do it. You need to clean the rust off and the dirt off. Nothing's going to stick over dirt and loose rust.