A B+ conductor is routed to one of the SPDT load
terminals, a B+ conductor is routed to the other load
terminals on the SPDT. The common (middle or source terminal) has a conductor leading to the B+ on the analog voltage panel meter. Select either battery
to check its voltage. Check for a blown fuse at connection to each battery or, in the case of the house bank, at its attachment to the panel board B+ bus.
Easy peasey, lemon squeezy!