When driving, my gauge read 18 volts. I pulled over and turned the truck off to check it out. I decided to try to make it home, and when I put the key in I got nothing. My lights and horn still work but everything that needs the ignition to be on does not. I've checked the fuses, fusable links, and replaced the ignition switch. Dooes anyone know what the problem is?

Sounds to me like an over-charge situation.
Too much getting past the voltage regulator may have damaged the ignition module. The lights and horns aren't as susceptible, so a better test would be to check for spark at the plugs.

And then, get the charging system diagnosed.

How do I check for spark if the engine won't turn over?
It may already be curtains for the battery. Batteries don't like being force-fed.
Have the battery properly tested. If it tests as good, you'll need to check for correct battery voltage to and through the ignition system: ignition switch to starter, etc.

The capn has you on the right track.