Battery Charging

My coach has a Triad-utrad convertor/charger, appear like the originals. When operating it only charges the house batteries, checked with a volt meter and got 13.57 volts at the charger and house batteries but only 12.57 at starting batteries. Does anyone know if this is by design? If the "battery merge" switch is "on" will it charge the starting batteries also and should this be done?

Anyone familiar with these charges? Are they a "dumb" charger (constant voltage) or "smart" and decrease the voltage as the batteries reach capacity.


Troy
1986 Newell 40'