Ford Repair: ford F600 repair quesion, battery cables, cubic inch engine

Question
Hi.  I need some ideas for troubleshooting an electrical problem on a 1987 Ford F600 heavy duty aviation fueling truck with a 6.1 liter 370 cubic inch engine.  We are having a problem with getting the truck to charge properly.  The meter is showing only approximately 12 volts at the battery and also at the alternator itself.  We have replaced the battery, alternator, and voltage regulator and have also had them all checked out to make sure that they are working.  We suspect that there is some sort of wiring problem with the truck.  It is impossible to get any type of reading over 14 volts in the system and consequently, the battery keeps getting run down.  Any ideas you may have for where to check now would be greatly appreciated.  Thank you.

Answer
John, have you replaced the battery cables and/or the hot wire?  You could be losing 2 or 3 volts by using old wires.  If you are getting approximately 12 volts to the battery, then it sounds like all the circuits are closed.  If you have replaced all the major parts involved in the charging system, then the only thing I can think is that you are using the wires and cables that originally came with the truck.  Try replacing the cables and check the voltage.  Then try the hot wire.  I hope this helps,   Jason