Michael, for 12V USB adapters, best to stick with those cigar lighter
USB adapters, because they're designed to have very low parasitic
current when not in use.  Every other form factor I've investigated
assumes it'll be powered by a 120V-to-12V wall wart, where parasitic
current is less of a concern.  The Blue Sea integrated jack Abel
recommends looks very cool, but I just use the adapters.  This is my
favorite, for its low profile:

www.amazon.com/exec/obidos/ASIN/B0042B9U8Q/sbw/

I try hard to rely on only 12V appliances and USB, even when connected
to shore power, for continuity of the "user experience" whether on
battery or shore power!  Like Abel, I have a ProSport 8, but I'm
planning to upgrade to an Iota 30A charger on the advice of Vanagon guru
Karl Mullendore:

www.amazon.com/exec/obidos/ASIN/B00A0V5HPC/sbw/

I have added several cigar lighter outlets with USB adapters, and wired
appliances, lights, and laptop chargers directly to a custom 12V fuse
block.  Details:

http://www.sbw.org/westylaptop/

You don't have to go that far, of course, but ...

> ... I'd rather run my toys on AC than battery if I can, but I want
> them hooked up to 12V so I have an option.  I initially thought of
> some kind of battery charger, but really I think that I would want
> more amps, and not to mess with the batteries if I can avoid it.

Your toys need the amps they need, whether batteries or shore power.
And batteries let you play when shore power isn't available.  I like the
flexibility.

> I'm thinking of a switching PSU from the computer industry, and build
> a new 12V bus--- I might be able to find a sub-ms 12V transfer switch
> so stuff won't reboot.

That sounds way more complex than a good charger and an
appropriately-sized battery.  Have a look at my battery installation
before you decide to go in a less traditional direction:

http://www.sbw.org/westybattery/