Look for a battery separator that wont connect the batteries unless there is sufficient charge available. Kind of like what's above the oil fill cap. Since somebody was in there removing the wheelchair lift I figure its a case of undo what was done last like what the guy disconnected keeping from getting fried by a few hundred amps. Clean all the connections to the right of the 6th picture with the oil filler cap. Since things did work one time I'm thinking loose ground wire floating around after the ramp removed or the relay handling when the battery is in line with the system.

If there is a local guy that sells the big batteries to wheelchair oriented vehicles and such try asking him for suggestion when talking prices on batteries or asking him if the battery is the problem causing lights not to work. Try to act innocent instead of like you just want free advice. Those batteries might be reconditionable.

I'd track down relays and fuses. There might be a swing down for the batteries so that a tray slides out with the batteries.

Oh yah. my van running 2 batteries and an internal generator has a switch for running the batteries in parallel. Might figure out if you have and arrangement like that and check if the relay works. If the big batteries are dead don't hold the switch just a short click so you don't fry your battery. Check water levels and try a long charge since deep cycles take longer to charge as far as I know.