ORIGINAL: motomike
I might be missing something, but if you are using an inverter, to convert DC to AC so you can plug in a charger and convert AC to DC there might be a better way to go.
I think most of the discussion has been about inverter generators, where the generator has a variable speed engine driving an alternator, that drives a built-in inverter with a pure sine-wave output.
You open up another option, though, in that you could use a regular generator to power a 12v charger hooked up to a 12V batt. The generator does not need to be able to handle the peak load, but just enough to keep the 12v battery topped off. The advantage is that you don't need to run the generator every time, only when you are going to be charging a lot of lipos. Most 12V chargers are simple transformers that won't be sensitive to 120v waveform distortions or transients. One of the Harbor Frieght 900w jobs ($90) and a 12v charger (15A output, maybe $50) and a 12V deep cycle battery ($75 - shop around).
Someone mentioned solar panels. It would take a lot of panels and they are not cheap. Rated output is only in direct sunlight, so doing much charging on cloudy days is not possible. I would really love to install a few hundred watts worth on top of my camper, but by the time I count the panels, wiring, brackets, charge controller and battery bank, I'm looking at probably $1500. It is too easy for me to pull out my small $150 generator to justify that expense.