the horn blows, the headlights turn on but the truck wont start

Tighten and clean all battery connections. Also tighten and clean the negative connection on the frame and motor. Is it an automatic or standard shift?

You say the starter is strong. Does that mean it turns over or it is powerful? If the starter does not turn over, start there. Check and make sure you have a connection to the starter when the key is turned to the starting position. If the starter turns, is there spark to the plugs? Is the timing set for proper ignition at the plugs? Do you have gas flow? FI or carb motor? Plugged jets in the carb or float bad. Not enough pressure to the injectors? A low battery will honk the horn and light the lights but not turn over the starter as it takes a heck of a lot more current to engage and turn the motor over. Make sure you have a full charge or try a jump start.

I'm having the same problem. Did you find a solution?

Coleman 13, have the battery tested. Then if it is at full charge, the starter, then the electronics.

Check the ground straps. They are uninsulated braded wires that are bolted to the frame near the starter. 1 is connected to the motor (usually the back of the right head) and the other connects to the body, On or near the firewall. it's pretty common that these wires will corrode over time and when they do they causes a voltage drop in the cabin that reeks all kinds of havoc on the digital electronics. Altough you would think that all the metal components being bolted together would be sufficient, it's not. These wires are critical for maintaining a stable voltage to the control modules mounted inside the dashboard. The anti-theft security module is particularly susceptible to these voltage fluctuations and typically will prevent starter relay from engaging and blowing a long steady Blast from the horn.