You are going to be much better off figuring out a circuit that will
charge your batteries without using the inverter.  The efficiency of
jumping 12v up to 100 ac then back down (with your charger) to the 7.5v
dc again to charge your batteries is going to eat a lot of juice.  Check
around.  Though I've never seen one (never looked) I'm willing to bet
that some of the RC model crowd has come up with chargers in the range
you need that are designed to run off a 12v auto system.  Or find an
electronics guru that can gen up a circuit you can put together
yourself.