The way these things actually work is you wire your battery to the converter. The converter senses what source to take power from or supply power to. That means it will charge your battery when needed when plugged in, but will not use the battery for power unless you are not plugged in.
Confused yet?
When properly hooked up these converters pretty much make power plug and play. You don't have to install manual override switches, separate fuse panels, wiring blocks, a charger and all that stuff. When I did the math it ended up being a lot cheaper then cobbling together an electrical system from components.
Some people will claim you need a high dollar converter to charge your deep cycle batteries properly. I have used the Centurions in several campers. They do the job and there will be a lot nicer wiring job in the end. You do need a bit of electrical knowledge to wire everything up correctly though.
This is a more appropriate teardrop converter for a far better price:
http://cgi.ebay.com/ebaymotors/NEW-RV-110-V-12V-CENTURION-POWER-CONVERTER-CAMPER-BUS_W0QQitemZ4551488102QQcategoryZ50070QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
Just remember to size the AC side properly if you are using an Air unit. Most of these have two AC circuits for this purpose. One fuse for Air and one for the rest of the stuff.