Wouldn't it be easier to just disconnect the alternator at the battery? I can easily travel 30 miles on a traditional lead-acid battery and 60 miles on a Group 24 Gel-Cel battery. Using a good battery charger, you can top off in four hours.
Here is the unit I use... the efficiencies are as high as 88% which means less kilowatt usage. Expect to use .65KW with each battery fill up:
Black&Decker 10 Amp Switching Power Supply Based Battery Charger
On the subject of the inverter and deep cycle battery.
You do not need an inverter and a battery charger. Get one of these battery switches, from your local marine supply store or online at WestMarine. With it you can connect your two batteries together. In the event of 50% charge turn the switch on and you will get the advantage of two batteries.
