Captain GD, my limited understanding of the differences between using 2x 6v vs 1x 12v is that it has more to do with the construction of the specific battery or batteries used and the DOD (depth of discharge) that the battery will be subject to than anything else. I think you are right in that the differences between using 2x 6v vs 1x 12v of otherwise identically constructed batteries may be a bit more academic than practical for most people's uses.
I lifted the following three paragraphs describing the attributes of various vehicle battery types from:
http://www.solar-electric.com/deep-cycl ... ry%20Types
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Starting (sometimes called SLI, for starting, lighting, ignition) batteries are commonly used to start and run engines. Engine starters need a very large starting current for a very short time. Starting batteries have a large number of thin plates for maximum surface area. The plates are composed of a Lead "sponge", similar in appearance to a very fine foam sponge. This gives a very large surface area, but if deep cycled, this sponge will quickly be consumed and fall to the bottom of the cells. Automotive batteries will generally fail after 30-150 deep cycles if deep cycled, while they may last for thousands of cycles in normal starting use (2-5% discharge).
Deep cycle batteries are designed to be discharged down as much as 80% time after time, and have much thicker plates. The major difference between a true deep cycle battery and others is that the plates are SOLID Lead plates - not sponge. This gives less surface area, thus less "instant" power like starting batteries need. Although these can be cycled down to 20% charge, the best lifespan vs cost method is to keep the average cycle at about 50% discharge. Unfortunately, it is often impossible to tell what you are really buying in some of the discount stores or places that specialize in automotive batteries. The golf car battery is quite popular for small systems and RV's. The problem is that "golf car" refers to a size of battery case (commonly called GC-2, or T-105), not the type or construction - so the quality and construction of a golf car battery can vary considerably - ranging from the cheap off brand with thin plates up to true deep cycle brands, such as Crown, Deka, Trojan, etc. In general, you get what you pay for.
Marine batteries are usually a "hybrid", and fall between the starting and deep-cycle batteries, though a few (Rolls-Surrette and Concorde, for example) are true deep cycle. In the hybrid, the plates may be composed of Lead sponge, but it is coarser and heavier than that used in starting batteries. It is often hard to tell what you are getting in a "marine" battery, but most are a hybrid. Starting batteries are usually rated at "CCA", or cold cranking amps, or "MCA", Marine cranking amps - the same as "CA". Any battery with the capacity shown in CA or MCA may or may not be a true deep-cycle battery. It is sometimes hard to tell, as the term deep cycle is often overused - we have even seen the term "deep cycle" used in automotive starting battery advertising. CA and MCA ratings are at 32 degrees F, while CCA is at zero degree F. Unfortunately, the only positive way to tell with some batteries is to buy one and cut it open - not much of an option."
When I bought my bus conversion last year I knew up front that I would need to replace the house batteries in getting it road ready again and fit for bookbinding. Knowing nothing yet about RV electrical systems, I just figured I would use 12v Costco deep cycle/marine/RV batteries. Fortunately the RV specialist in MN guiding me on the 4 month bus prep project had me replace the existing sixteen 6v Trojan golf cart batteries with the same. I have since found I can run both rooftop AC units at the same time for a hours at a time while driving using inverted battery power and engaging the battery merge switch which connects the starting batteries with the house batteries (both are 24v systems). When in camp, when the house batteries are charged I can run both ACs on battery power alone for a couple of hours at least (without the battery merge switch engaged) before the inverters want to auto-start the generator.
Btw, the house electric system consists of a 50 amp shore power connection, a 8kw diesel gen set, four banks of four 6v batteries, two Trace SW4024 inverter/chargers, and various devices like a battery equalizer, etc.
For some reason I have always struggled with understanding electricity and electronics. I have much left to learn so I can diagnose and repair my buss' electrical systems anywhere near as quickly as I can address the mechanical, hydraulic, pneumatic issues that inevitably arise with a well-used vintage vehicle.
I hope this answered your question. The link above seems to be a good resource for knowledge on these subjects.
