Think out side the box...Large trucks use 12v for every thing too, but some have a 24v starter. How can you run a 24v starter on a 12 system? 
This is kind of true. Most big rigs are a base 24v system. The alternator, starter, engine(ecu and sensors) and many other things are 24v parts. They do have a 24v to 12v dc-dc converter (which is a lot less expensive than going 12v to 24v) for all the standard stuff such as cigarette lighter power outlets and lights (some even have 24v lighting systems). I state this based on my own experience with my father's rig, which is several years old. It wouldn't surprise me if the newer rigs run more off the 12v system, since the price of 24v to 12v converters has dropped a lot in recent years, but they still must use a 24v alternator to charge a 24v system.
A second idea, is to switch the alternator and batteries to a 24v system but run an A/B POWER system. The only possible problem would be that both batteries share the same ground...
which could short the second battery to the first. A and B have to be ISOLATED from each other.. so this may not work...
This is actually a pretty good idea to make this happen, only you don't have to keep the grounds isolated from one another. You just put in a 24v to 12v dc-dc converter to run all your normal 12v stuff. The converter is fairly cheap, being that it's just a voltage divider. The more expensive parts would be the alternator and the new guages for monitoring your battery voltage (you would want to monitor the 24v level as well as the 12v level). The only caveat would be the current load that the 12v starter would put on the dc-dc converter might cause some problems.
Moving on to another (similar) solution, you may not even need to use a dc-dc converter if you run 2 12v batteries in series (24v) and a 24v alternator. Just tap into 1 battery for your 12v stuff since each battery is putting out 12v. It will be a voltage divider in itself. The important thing here is to make sure you do it right and check it twice.
See this link from a guy with a similar situation for some more detail on this method.
http://www.thetruckersreport.com/truckingindustryforum/car-and-truck-talk/118628-high-amp-24v-to-12v-conversion.htmlGood luck.
