Larry is right on batteries from Middlebury.... 4 is the max to the best of my knowledge. However, one of the regular participants on this forum is Rubicon Trail and he installed two extra L16s in his 2018 Cornerstone. His thread on that subject is on here. Use the search box up near the top right (Google) to search this forum and you will find it.
As I remember it, he had to move the partition that covers the batteries out to accomodate the additional batteries with a loss of basement space, but then simply inserted them in beside the stock batteries.
Gary and Dee, Zowie and Bowie (traveling cat sibs)
2019 Cornerstone 45B, X15-605hp, Imperial, Spartan K3,
2013 Honda CR-V toad, Demco Excali-Bar II,
Demco Baseplate, Demco Toad Light system, 73 de W5Fi