E 350 7.5 gas quandry

[philip s]

Good morning All

I have two E 350 super duty full size chateau's both have a dual battery set up and both display the same issue. I have left the dome lights on or whatever and run the primary battery down to zip,the second battery is at full voltage but the entire truck is dead electrically. Does anyone know what that battery is for? The factory manuals are not as forthcoming as I would like.

Phil

 

[Mil1ion]

Usually for RV type applications...Trailer wiring ?
or

Van Accessories.

It doesn't show much but here is a schematic.