Engine alternator only charges engine battery

We have a 2001 Itasca Suncruiser Ford F53. Has an engine battery and (2)12 volt batteries. When plugged into shore power or when the generator is running, the 2 12 volts are being charged. But when the engine is running only, the 2 12 volts are not being charged. I know on our Roadtrek campervan, the engine alternator does charge the accessory 12volt battery, but in our Itasca Suncruiser, it does not. Is this a design issue or is there something wrong??
I'd sure like for it to charge those 2 12 volt accessory batteries when driving. Any help would be appreciated.