I'm doing something similar, but with a different strategy:
http://ecomodder.com/forum/showthrea...tml#post384677
I haven't looked, but I am sure a 12V DC-DC converter capable of car level amperage is going to be rather expensive. You really have to know what demands your car has, and make sure your DC-DC converter can supply more. Also, DC-DC step up conversions increase voltage but you loose some current in the process. Be sure to know exactly how much current is reduced at the output. It will factor in your calculations to figure out how long the deep cycle can charge and run your system.
The only reason (that I can think of) to stay at 14 volts is headlight dimming. The cars electronics will run fine on 12V. Deep cycle batteries often have sufficient CCA ratings to start our little engines, so that isn't a concern unless your under extreme demands on starting. You'll have to decide if the dimmer headlights are still acceptable safety wise. For my car 14V vs 12V is a noticeable difference, but just because you notice the dimming doesn't mean its unsafe. Faded lenses and old bulbs are more of a safety risk than running headlights on 12V in my opinion.
There is also problems with running multiple batteries in the car. For your system it's a bit different so it may not apply. If I were to simply connect a deep cycle in parallel with my starter battery I run the risk of damaging the starter battery. Those batteries are not designed to discharge much at all. If I run them in parallel they share a voltage and combine capacity, but I am still discharging it more than I would want to everyday. This could cause it to go bad earlier, further making my system inefficient and costing me more in batteries. Having a single deep cycle, or multiple deep cycles in parallel provides a more flexible setup with discharging.
Overall I think cutting out the alternator could yield good MPG results, but it probably won't be economical overall. When you factor in the cost of batteries, the cost of the extra components, and then the cost to charge the battery from your house, there is very little room for any actual savings. But for me it's not all about actual savings. Some of it is just seeing how high my MPG number can go, regardless of what it costs. If that is you too, then go for it.