even though you have correct _voltage_ to the contactor coils, the current can be weak, if you have bad switches in the circuit, that are breaking down under a load.
I would recommend taking a set of jumper wires to the contactor coils (remove the wires from the coil to isolate it from the truck's circuits) to "eyeball" the operation of the contactor, and see if it is mechanical binding, or a "weak switch*" before it in the circuit. if it pulls in correctly when jumper-ed, then it is not binding, but is likely to be a switch that has a buildup of corrosion on the contacts of the switch. (my first switch that I would test would be the park brake switch if it has one, then the steering switches, but) any switch in the circuit could be the cause.
Since you say; "have a service manual to know where and how much voltage should be seen" I trust you fully read and understood the part that discusses "before troubleshooting" that is in the manual [before the troubleshooting section], that says to carefully check for "shorts to frame" on all power cables, and too fix any 'short to frame" of less than 100 K ohms, and you have (of course since you have the manual) checked and have clean connections when the battery is disconnected and out of the truck.