Hey Walmart I know................

No isolator, they are in direct parallel.

Isolators, unless there has been some huge scientific change, do not "charge one at a time." Most isolators are a pair of diodes, and can be dangerous. If the diode to the main opens, the main won't charge but the sense wire will want the alternator to ramp up as it thinks it's dead. So the spare battery will get boiled

I recommend ONLY to use continuous duty solenoids for second batteries such as campers, and if you run the main down somehow, you can jumper the solenoid and use the second to "jump" the main.