Brian, the problem with a smaller bank is it cannot handle heavy loads, such as a microwave, etc. The peukert effect especially effects 6 volters; although they do bounce back well, but it can set off the low voltage alarm on your inverter.
I will let someone else deal with your wiring set-up, as I built my own camper, and do not have distribution panels and such.
As for how long it takes to charge a pair of 6's. That depends on several things... Ah capacity, charger size, depth of discharge, voltage setting, etc. Roughly speaking, you can do a 50-90%, at 14.8v and a C/4 rate, in about 2 hours. Most people like to keep the rate (and voltage) lower, but there is no evidence I know of that suggests faster charging will hurt your batteries. And even if it does take a year or two off your batts, it saves so much in genertor run time and fuel, it's well worth it. Of course, going solar, you'll need the gennie much less. So you have to consider your needs, camping habits, budget, etc.
Just keep in mind, converters can be very slow at charging, because many drop down to 13.6v during the constant voltage / absorption stage. So just as the battery's acceptance is dropping, the converter's ability to "push" amps into the bank is significantly reduced at the lower voltage. 2-3 hours can turn into 6-9 hrs... not good when running the gennie.
I noticed you said 2-6's wired in parallel. No. You wire two in series for 12v. Two such "strings" can then be wired in parallel to double Ah's.
http://www.trojanbattery.com/Tech-Support/TechologyLibrary/ConnectionsDiagram.aspx
