I have a 2019 AF990 and it will hold (2) group 29 L/A battery's as stated above.
I removed mine and added 560ahr of Lifepo4 in the same compartment that includes a 200a BMS Fuse block and Pos/Neg terminal boards for all connections. Even have a 1/4" foam box built around the battery's to help insulate them from cold weather and for protection as well.
Got 510w of solar on my roof also and even have a 3k PSW Inverter mounted back in the generator compartment for any 120v loads we need.
I could of removed my sliding tray or even Modified/Cut out a portion of the battery box but didn't need to as it all fit fine.
Don't really have to worry about any gases with Lithium batteries so they don't necessarily need to be in a closed compartment.
We go off grid quite a bit and never really have to worry about having availible power.
Congrats on your new rig.
