E-350 dual battery set up

I have 1993 E350 /460 super duty passanger van that has a dual battry set up. The question is what does the little second battery do? when does it kick in. I let the van sit for about a month not long ago and the primary battery went dead ,or at least dead enough to not start the truck. The "aux" battery was still reading 12.4 volts but the lights (interior and otherwise) were all dim or out.
The factory manuals wiring diagrams are not giving up much help. Can anyone shed some light ( as it were) on this.

Phil

If the van was designed to act as a camper, then 2 batteries are installed . One battery is to power accessories in the camper, the other is to supply power for the starting and operation of the vehicle. They are hooked together in sequence so the alternator can charge both as you are driving.