Having a "second aux battery" would be a nice factory option, so you don't worry about draining the starter. There are mods you can do to vehicles to make this work.
I'd like it in an SUV, and I'm willing to bet if offered, folks would jump on it, but I agree it's a lot less likely it'd be heavily used.
Re: Adding another battery, I've considered (and done some napkin math) adding solar panels and a second battery to my car to power the computer I run onboard. But it's something I don't have the time or money to play with right now.
Get the specs of you generator and you can see how much you can sustain while your engine is running. The inverters do usually create a square voltage "wave" rather than a good sine one, but it has never been a problem in my experience.