On a standard Knaus Sun Ti 650MF, there is NO switch, to select between starter/habitation batteries.
The charging/distribution circuits, are controlled by a Calira system. Unfortunately, there have been numerous issues, with this, for many Sun Ti owners.
Based on what you have said, I suspect, that the Calira, might not be charging, and, if this has been going on, for some time, there has been a steady deterioration, of both batteries, because they have only been receiving a charge, whilst driving. Also, depending on the batteries age, they might well be past their best.
If it is the Calira, then replacements are quite expensive, but they can be repaired, but, again, not cheap!
A number of owners, are now on their 3rd Calira. In my opinion, they are NOT robust enough, although a well known expert, disagrees, and blames the owners, for causing their failure.
Instead of renewing/repairing, a Calira, I would recommend replacing it, with a more robust charging system, but retaining the Calira control system, so that the "Bordcontrol" panel, would still work. I did this, myself, after 2 Calira failures, and everything has worked ok, for the last 8 years.