[72lb4x4]

First, measure voltage directly across the circuit. In this case, the 2 battery terminals.

12.6V is normal on a good charged battery. If you're getting 12.5V it may be that the meter isn't exactly accurate. A no load voltage test on a battery doesn't indicate its good, just that its not necessarily bad.

A new battery isn't necessarily fully charged when you buy it. Its probably mostly charged.

Do you have a charger? I'd put the battery on a trickle charger for a few days and try again. Look into a battery tender if you aren't driving the truck regularly for a decent distance to keep it charged.

You say the voltage is down to 11.9V with the engine running. Not good. Try reving the engine to see if the voltage comes up above idle. You should get at least about 14.5V.

Quick tests for the big cables if the starter spins the engine slowly are to use one half of a set of jumper cables to connect the - battery terminal to the block, or the + to the starter and try to start.

Since the starter spins the engine, I wouldn't be too worried about the ignition switch or solenoid just yet.