[Hemlock_SF]

I have a new to me 2005 Tiara 3800 with 7 West Marine gel batteries with a K5 Sticker on them so i am assuming 2015 battery year. Since the manual is missing the pages for the parallel wiring diagram i am guessing 3 for the house, 3 for the engines, and 1 for the inverter / charger? Anyway i found the charger wire for the engine group going into the C-Charger barely hanging in there and probably not charging very well for quite some time. I have re attached the charging wire and the 3 engine batteries do not seem to be coming back to life, they are currently sitting at about 6-8 volts after 10 hours of charging, all other batteries are sitting at 13.5 volts. So my dilemma is do i change out 3 gel batteries for $1500 or buy 7 new AGM batteries and a new charger for $2k? The C charger does not have an AGM setting but the C-charger alternator charger does. Any input would be greatly appreciated.

Also wondering if anybody has had issues with the battery monitoring not being accurate?

Actual battery voltage was around 6-8 volts 
3.5 volts on the meter
13.5 on the digital meter