We just recently replaced the 8 each 8D house battery bank in our Eagle at a cost of approximately $3500. We also have 2 each 8D wet cell batteries for starting and the bus' 12v lighting system.
There is a HUGE difference in the care and feeding of the two different banks. The start bank requires regular cleaning and topping off while the house bank is pretty much totally self sufficient. As Tom says, if you haven't experienced the freedom of using the AGM battery, then...
It is my opinion that over the long haul, the AGM outshines its wetcell counterpart in every department - including cost.
Also, I agree that having two seperate battery banks is 'penny wise and pound foolish'. The house side should never be discharged past 50% in any case, which leaves a more than sufficient amount of cranking amps to fire the engine.
I'm also considering building a house battery system (24v) and combining with vehicle start system (also 24v). Garhawk- You suggest a huge difference in "feeding of the banks". Would changes need to be made to the vehicle charging system?