Engine not cranking over?

Hi aquastell. Welcome to the forum. I think you're getting "wrapped around the axle" and way over-thinking this. Your dandy description of the symptoms is exactly what happens when you have a dirty or loose battery cable connection. Due to the high resistance causing the low voltage, computers on the truck will do weird things such as that horn chirp you heard. Common sense says a computer is not needed to honk a horn, after all, we had horns for decades without computers, but common sense doesn't prevail anymore on newer vehicles.

Since this problem is acting up regularly for you, that will make it easier to find. A cheap digital voltmeter can be used but in this case, a test light would be faster. The only thing that can't be checked on GM vehicles is the battery connections because of their side post terminals. Start with the test light connected to the two battery cables. If the test light is dim there, the cable connections must be cleaned and tightened, or the battery has a bad connection internally and must be replaced.

If the test light is nice and bright, move the probe or clip lead from the negative post to the engine block. Also move it to the body sheet metal. There will be a smaller wire attached to the fender that comes from the battery negative post. The light must be bright at both of those places. The wire to the fender is the return for all of the lights, the horn, the starter relay, etc. It is a common source of a bad connection, especially if the inner fender is getting rusty. Next move the probe or clip lead from the positive post to whatever the cable goes to. On older vehicles, that was the starter. Its terminal was used as a convenient tie point for other wires to branch off from. On newer vehicles, look for an underhood fuse box with a smaller cable going to it from the positive battery post. Move your probe to the connection on that box. You're looking for the point at which the test light becomes as dim as the other lights on the truck. When you find that point, that's where to look for the loose or corroded connection.

Caradiodoc

Dandy. Your truck had side post terminals originally, but the top post design makes it easier to test. Look at every mechanical connection to be sure they're clean and tight. You can use your voltmeter to measure across each connection. Start with the negative side. Turn the headlights on. When the problem acts up, put one meter probe on the battery post and the other probe on the cable clamp. You should read 0 volts. There will always be a very tiny voltage drop, but the closer to 0 volts, the better. Measure across each connection that way on the negative and positive circuits. One of them is going to have a very high voltage reading. That's the source of your problem.

Caradiodoc