My 1999 320 had a dumb charger and I replaced it (previous owner was on a mooring and never used it). It was located behind the galley bulkhead in the port lazerette. I put my new charger in the stbd side locker in the v-berth to be closer to the batteries. The boat came with two 4D AGM batts replaced in 2015 so obviously not original. I'd be surprised if your batteries are original, but not surprised if they are FLA.
So everybody understands the layout. The batteries are all located under the stbd settee in the saloon. It's a big, open space with 3 separate lids covering the 3 large openings. There is room for 2 4D batteries and a GRP 27 auxiliary battery plus a lot of space for the holding tank, hoses and whatnot. Cushions cover the lids so ventilation is very limited. The batteries sit in trays, so they are in open air within the compartment. The compartment is open to the bilge, but obviously that is lower level. Bilge covers are not vented but it is open to the compartments on the port side. I wouldn't say any of these compartments are vented, but they are relatively large open areas underneath the cushions. The area is also open to space behind the head area. There is a lot of open air underneath the furnishings.
First thing I would check is to find out what charger you have and most likely make an upgrade. If you actually have FLA batteries that are not sealed, I would think about an upgrade there, too. Obviously, I would consider the current health of the batteries.