Insulation is achieved by trapped pockets of dead air.
Air being re-circulated through the plastic wouldn't do this as well, as far as I know.

 I can see a cooling effect, and In greenhouses, there is usually fans or openable panels to keep the temperatures down in the summer.

 but i can't see an advantage of having moving air between two sheets of plastic holding heat in, I'd think you'd be better off with two sheets separated by the frames, and taut.

 Of course, this isn't my project, and you already have the greenhouse, and you just want to know the best way to use the fan.

 I'm agreeing with most that using an inverter, vs using a DC fan that is capable of the same CFM of air movement would be wasteful.

 Every step in conversion of energy looses something. Your battery bank will be the biggest loss, however the sun doesn't shine all the time, so you do need storage!

 Also is the total cost of all the equipment over using grid worth it? It looks like to me,, there would be a LONG payback period.

 Maybe you would be better off just using grid electricity, and putting your money into something that would give you a better payoff. (such as solar hot water)