Are all 5 batteries 12v models? if they were connected in Series, you should have 24v and not 6v.
Basically, hooking in Series doubles the voltage. Hooking in Parallel doubles you amps.
For Series, Batt 1 has Neg. connected to Block Ground and Pos. connected to Neg. of Batt 2. Batt 2 has Pos. connected to Starter (or wherever it goes on yours). if you connect a volt meter to the block (or the Batt 1 ground point) and to the Starter (or Pos. connection on Batt 2), you should read 24v. if it does not, you my friend have an issue either with connections or the batteries.
For Parallel, Batt 1 had Neg. connected to Batt 2 Neg. and Pos. connected to Batt 2 Pos. Batt 2 then has Pos. and Neg. connected normally with Neg. connected to Block and Pos. connected to Starter (or other connection point). With this setup you should be reading 12v on Batt 1, Batt 2 or at the Ground and Pos. connection point. If you are not, you have an issue here with either connection or batteries.
Now if you did have battery issues and there was no reserve in them, engaging any power hog could stall an engine due to the load placed on the alternator to supply that voltage. It would take a lot to do it but it is possible.
So how old are all of these batteries. As a general rule of thumb, a good battery will generally only last 5 years or so. Sometimes you will get lucky and get 6 out of them but that is about it. When I mention "good" batteries, I am talking about Interstate, Duralast Gold, and some others. Not your bargain basement 69.99 battery sold at your favorite bulk store.