I chose the 2 6v batteries(which are wired in series) because the load at a 15 amps is about 28 hours to wipe out the coach batteries as compared to the standard 2 Group 27's that are about 11 hours. Of course, temperature and battery age can change performance. The 6v are more expenisive also, but I've seen them as low as $110 each. It is my understanding that any deep cycle battery REALLY shouldn't be discharged more than 50%(12.2v). (That figure doesn't count if a load is being applied)
On the other side of the coin, what you take OUT of the battery needs to go back in. I think the Ford chassis has a 130 amp alternator and the house charger is about 60 amp. I read somewhere in another forum that it is actually a "break even" on fuel consumption charging with the V10 at high idle vs the genset/house charger. The V10 uses more fuel but the amps get dumped in the batteries MUCH quicker. Depends whether you need the genset for other things...
Ron, have you load tested those Exides to see how that stand up? You can take them to any Advance Auto and they will check 'em for you. If they need replacement, its best to stick with real deep cycles made by Exide, Interstate, Trojan, etc.