If it comes with a diesel genny, that's great. They cost a small fortune to buy (the ones designed for the leisure / marine market, as opposed to those designed for a building site).
You're right, you don't want the thing roaring away late at night... for your own peace and quiet as well as everyone else's! Besides, running a ~10kW genny for a couple of hundred watts of TV and lighting is not at all economical! Also, if you have a standard domestic electric fridge (mains powered), you don't want the genny kicking in in the middle of the night to run the fridge compressor (some gennies have autostart & load sensing)
I'd recommend that you get an inverter-charger. This will charge your batteries when the genny's running and provide silent mains power when it's not. The charging circuit's very efficient and can charge your batteries to a higher capacity than an alternator (unless an alternator controller's fitted).
I almost bought one myself, but having decided not to have a genny (landline hookup means that I'm not that bothered that the batteries don't get optimally charged) I saved the money.
Have a look at the products offered by Trace Engineering (both marine and RV). The DR series is quite reasonably priced for the piece of kit it is. They are an auto-sensing inverter-charger and may be more suitable than the ones specifically tailored to RVs, it depends on your generator - there's no point in having one designed for auto-start gennies if there ain't an auto-start, for example
There are two types of inverter - true sine wave and modified sine wave (MSW). Most equipment will run off MSW - my computer and TFT screen are fine - there's a slight bit of rippling on the monior, but nothing irritating. Some printers can object to MSW. Regarding anything else - it's a bit of a gamble - I don't have any other experience than that. There is quite a big difference between the cost of a true sine and MSW inverter, so if you can find out whether all the equipment will run on MSW, you'll save a good few hundred bux
If you plan to fit aircon (electrically driven compressor) then I reckon you'd either need the mother of all battery banks, or just run it directly off the genny. Ditto for anything like washer/dryers dishwashers - they just use too much power for battery banks!
I've got another link for you:
http://www.heartinterface.com/
I think, once you know what kit you'll have in the vehicle, you'd do well to contact one or more of the inverter manufacturers and see what they recommend - ask em about battery sizing, too