On board charger ?
I had a similar set up -- 3 batteries and a two bank charger. My dealer wired things so that the charger would charge all three batteries. One bank charged the cranking battery and the other bank charged the TM batteries. Obviously, the charging time for the TM batteries would take longer than with a 3 bank charger. But, in an overnight situation, all three should be at full charge by morning. This has worked just fine for me for about 10 yrs. now. Just an alternative to buying a 3 bank charger.