I added 4 additional AGM batteries to our 2014 CS Adventurous. Since the existing batteries from Roadtrek are AGM, the installation was straight forward, with the new batteries wired in parallel with the others. Note that the original coach batteries are 6 volts and are connected in pairs to make 12volts. This doesn't need to be changed, just FYI. The sprinter alternator would handle the extra batteries as it is controlled by the Sprinter voltage regulator. Our Sprinter had a somewhat bonehead method of circuit protection that resulted in strange noises under the hood and batteries that would not charge when discharged after a couple of nights of boondocking. This was Roadtreks design, not MB.
This setup has been working well for six months now, but I would not use this method with batteries of different chemistry types (lithium) and ages, so you will have to do some homework. IMHO the lithium batteries are not well suited to cold climates and need to be maintained and kept warm. They also have different charging requirements than lead-acid or AGM batteries. I have also added an underhood generator (alternator) that looks after the (8) coach batteries now, but, as they say, that's another story. Hope this helps.