How you wired the batteries was one problem and pairing different age/brand batteries could also been a problem.
Suggest you Google "the 12 Volt Side of Life"
It will teach all about the 12v system and properly wiring batteries.
And get a multimeter.
Harbor Freight has coupons for a free one.
and Zoe the Wonder Dog(R.I.P.)
'07 Roo 23SS pushing an '07 Chevy Avalanche
Equalizer WDH and Prodigy BC