On our 2017 CY I first wanted to mount the 400AH Lithiums under the bed to make room for all the solar components in the electrical compartment. Two problems were the longer cables and the bed doesn't lift....so I nixed that idea totally and installed them in the factory spot. I could still add one more 200AH battery next to them, but it turned out that we didn't need the increased capacity for the amount of solar I put up.
I did get all of the extra add-ons in that compartment by adding a piece of plywood on the separating wall of the generator compartment...all discos, 3K inverter, fuses, and auxiliary positive/negative multi-hookups, etc.
The only item that wouldn't comfortably fit in that bay is the solar charger....I put it on the pass thru compartment wall up high and out of the way for a short wire run to my +/- attachment points. Also cut a hole thru the compartment next to the charger for a 24/7 running of a 5" computer fan to evacuate the charger's heat produced from it's heat-sink when batteries get to 100% on those hot days.
It all fitt and I can still get my head into the "electrical room" to work on everything..