The flexible battery plan is what I used way back when. I had a clamp steel battery carrier and jumper wire clamps and put the batteries in the bed. Twisted a marine switch from 1 to ALL and charged the batteries 2-hours on shopping trips. The old Chevy had a 130-amp Leece Neville alternator and I spun up 14.8 volts on the L79000 voltage regulator. The batteries started off at close to a 90 amp charge rate. A bit of work involved but that was before afforable solar. Thought my Honda gen and 50-amp Associated charger was hot stuff - 40-years ago :)