Got my new (used) batteries!! Now, how do I rejuvenate them?

your NoCo is fine, short length 18 ga will carry 10 amps no problem

NEC ratings are For wires hidden in walls, running thru pipe, hundreds of feet
where heat can not be allowed to build up and cause a fire

in RV battery power use, its all about avoiding voltage drop to the batteries or to the inverter, much smaller wire could be used in most cases regarding the amps involved and be perfectly safe, it just would NOT be as power transfer efficient because of voltage drop