I'm running basically an older version of the same thing. Yes, these things are the cats meow. You're correct in your assumption that it serves as both the 120 volt A/C load center and the 12 volt D/C load center. AC and DC are physically and electrically isolated. On mine, the AC side uses standard household circuit breakers (1-20 amp circuit, and 2-15 amp circuits in my case), and the DC side uses automotive fuses. I assume most of these work the same way. It works well. The AC side works exactly like a household circuit breaker. Your circuits should be wired essentially like those in your house (with a few exceptions). The fact that it's an AC/DC charger/converter means that it also provides a lot of DC power to your trailer for things like lighting, refrigeration, and fans, all while charging your battery.
There are a few more things to keep in mind:
These units are capable of drawing way more current than a standard household circuit can provide. They're made to use with RV style service. In order to utilize your capacity, your shore power inlet needs to be a 30 amp RV service style plug. And your shore power needs to meet that capacity. You can use an adaptor to hook to a standard household 15 or 20 amp as long as you don't need the extra capacity in your trailer.
This is what I use on my trailer:
http://www.amazon.com/ParkPower-301ELRV ... GZPTCSJN8JThis is what I installed in my trailer-parking area:
http://www.homedepot.com/p/GE-100-Amp-3 ... 393689-_-NAlso, pay attention to the type of DC charger the unit has. Mine is an old fashion analog charger/converter. I'd like to buy a newer smart charger model that's capable of charging my AGM battery safely and efficiently.
Have fun!