Battery interconnect logic

If the battery combiner relay is engaged when travelling, the solar is already being added to the alternator's power (as the two are combined), so there's no need to do anything additional for that particular use.

There might be some circuit that senses somehow when the converter is running and/or possibly when the solar is charging and combines the two batteries at that point, too, to make sure the chassis battery stays charged. Typically that would be part of a larger charge control unit or a fancier relay generally termed a BIRD (which stands for bidirectional something something device, I think).

A picture of the relay with the wires might be helpful. I can't think of any reason to have four control wires for this one relay. Usually there would be just the heavy wires on either side to either battery, a single control wire, and possibly a ground connection if it isn't grounded through its mount.

The control wire, in one typical and relatively simple setup, goes to the emergency start button, which is a SPDT switch. The normally closed contact goes to the run circuit on the chassis, or something similar, and the normally open contact to the house 12V system somewhere (via a fuse, etc.). There are fair few possible variations; that said, what I've described is how my 1998 Coachmen is wired up.