Regarding use of a large capacity alternator to better tap power from an idling main engine.
It might be a better approach to use a couple of moderate capacity alternators - such as a couple of 200 amp ones - instead of just one huge alternator. IMHO, it's always better to err on the side of redundancy when it comes to RV systems. For example, Earthroamer expedition RVs use twin large capacity alternators instead of only a single large alternator.
For charging large RV battery banks from either a single or twin alternators, I like the looks of these units:
http://sterling-power-usa.com/12volt-210ampalternator-to-batterycharger.aspx