First, you need to know what series & parallel is. If a battery cable from one battery connects (negative) to directly to the positive of the other battery, then this is series; and you are cranking with 24 volts. ... Parallel connection is when the positive cables & negative cable, each, connect to common points. This is 12 volts.
Looking up the starter on the internet, the P30 Chevy starter is shown to be 12v. So the batteries should be wired parallel.
Now you can test the starter. At the starter solenoid you will find a large battery cable connected. Check the voltage to make sure it is 12 volt. Then with a screw driver jump from the bat cable to the small wire connection. The solenoid should close and the stater start cranking. MAKE SURE THE TRUCK IS IN NEUTRAL AND BRAKES are LOCKED. If the solenoid closes but the starter does not crank, the starter is bad. If the solenoid clicks, then repair the solenoid. ... In the solenoid are two brass contacts. These wear out and do not make good solid contact. You will hear a single click when the solenoid closes. If you hear multiple (continuous) clicks, the solenoid need replacing. http://shop.advanceautoparts.com/webapp/wcs/stores/servlet/product_x_5220459-P_x_x?cm_mmc=ACQ-_-Google-_-GPLA-_-5220459&ci_src=17588969&ci_sku=5220459&ci_gpa=pla&ci_kw=&gclid=COb-5dyHproCFcuZ4AodgikAMQ#
If the starter is cranking, go back and investigate the 'doo hicky thingy on the fire wall'. It must have a small solenoid built in or be another solenoid. Check and make sure all the connections are tight. You should be able to test it the same way as the stater. Locate th power lead and jump to the starter lead.
It the batteries are full charged, you do not need the charger/booster to be connected.
<message edited by edwmax on Sun, 10/20/13 3:20 PM>