Paul:
Your report of low voltage at the lights with more than 1 fully charged 12 volt lead acid battery (you did use the term "BATTERIES"), says to me that you have a poor connection somewhere in the current path to the lights. This could be on either the positive wiring or on the grounded negative wiring. A quick beginning step to see if the low voltage is the true cause of your flickering lights is to run some temporary wiring directly from the battery to the lights and see how they work. Next try the lights with the old positive wiring and the temporary negative wire to see if the original negative wiring is bad. If you find you can trust the original negative wiring, then you have to trace out the positive wiring junction by junction until you find the bad connection - the place where the voltage with the lights on jumps from 12 volts to 9 volts. A hypodermic needle attached to your DVM lead is an easy way to pierce wire insulation and take readings along the wiring. This individual wire tracing can be miserable and another reason to give the job to a professional.
2 fully charged lead acid batteries have a LOT of amperage capacity (think of the emergency welding repairs back country 4 wheelers do with 2 lead acid batteries) and any fault that causes them to discharge rapidly should be generating a LOT of heat. Your light circuit SHOULD be limited to about 15 amps by the fusepanel, though.
Good luck!!!
