Hey! I converted my 2012 Express to a campervan and am trying to hook up my aux batteries to the alternator via a battery to battery charger. Where would I connect the positive line going to the charge controller?

 

the chassis battery, or the alternator, or the starter, depending whatever is convenient/closest to your controller.

The controller must have come with instructions

 

Normally an RV use a battery isolator : This device allow you to charge your chassis AND aux battery while driving. But when the ignition is turned off it disconnect the chassis battery to the rest of the circuit preventing
accidental discharge.

It is normally located under the hood where a large gauge wire run to your aux battery.

 

Im using the smartpass battery charger system to manage solar charging and it's also able to charge my aux battery via the alternator. The schematics on the unit just have a wire going from a terminal on the Smarpass D250SE to the positive terminal of the battery. I'm assuming there's a proper power tapping spot (rather than just splicing into the red wire coming out of the battery), but I don't know where that is...

 

 

According to the instruction manual.... if it is the correct one ? SmartPass120 ?

Then yes what MMJoe is showing you is what you need. Make sure the wire gauge is big enought and dont skip the fuse part.