As booster and hbn7hj have stated, you can definitely add a DC to DC charger that isolates your coach and engine batteries from one another yet charges your coach batteries off the engine alternator. Limiting current to 40A is a good idea and would be adequate to keep coach batteries charged without over-taxing your engine alternator.
However, have you verified with Coachmen that your rig doesn't have this feature? Pretty standard to have alternator charging. In fact, I've not heard of a modern rig that doesn't. If you're not getting charging now, could there be a loose connection or blown fuse?
If you can gain access to your power center (where your inverter resides) and post some photos, I'm sure someone here can identify if you already have an isolator in place.