FW has incorporated into their class A motorhomes what they call  "Dual Battery Charge Control".  The following is correspondence (minor editing) with Jim Pourroy at FW Training Center regarding just how this capability is achieved, the components involved and their location in the unit.  (The location will vary by model but the description should help lead to them.)

 Leo,
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> Sorry for the delay in responding, I have been out of town training.  I have always gone with the 217 Ah rating in all of my computations.  Trojan is the final word and you can go to their web site www.trojan-battery.com for the most accurate info.  I believe the T105 model designation is also a rating, 105 minutes at 75 Amps.  When I went to the web site to double check I found that I was wrong, it is 115 minutes at 75 Amps.  They also provide a rating of 447 minutes at 25 Amps which would be closer to a motor home application. This also goes to prove that amp hours are not an exact science as they vary at different rates of discharge.  You can't get 1 Amp for 217 hours or 217 Amps for 1 hour.

About the battery control center, it does indeed have an isolator that is intended to be used to charge the batteries from the alternator or inverter charger.  The way it is designed to work it activates the isolator relay (solenoid) to link the batteries when either charge source achieves a voltage of 13.3 or greater.  The activation follows a 12 to 20 second time delay.  The relay is intended to remain activated until the voltage drops below 12.6 VDC.