Re: Battery Sizing - am I crazy?
"but at twice the cost is the benefit twice as much"
I think you may be misunderstanding the numbers. 6 volt batteries will have twice the amp-hour capacity of a similar size and cost 12V. When you wire two 6V's in series you get a bank that's very similar to two 12V's in parallel in every aspect including size, cost and weight.
Where the advantages come in are mostly in overall lifetime. Golf Cart batteries are designed to be truly deep cycled and last longer than "Marine Deep Cycles" in this application. Marine batteries, even good "Deep Cycle" ones still compromise lifetime for cranking amps. This is unfortunate and leads to lots of confusion over this issue (see my posts earlier on in this thread..)
Just to be clear, the advantages of the GC batteries have little or nothing to do with the fact that they are 6V. It's that they are one of the easiest to find suitable true deep cycle batteries and the fact that they are 6V is a coincidence. As Mainsail has pointed out, full deep cycle 12V's exist but are harder to come by. By itself 6V is actually a small disadvantage because of the added wiring complexity. But the longer life, increased capacity, low cost and often better form factor (taller and narrower) of GC's often make up for this.