I am getting ready to mount a large invertor~3500watt nominal and 6000+ surge.
700watt isn't much,divide it by 120v and you'll see about how many amps you can pull~ 5 amps.You might want to look bigger.

Behind the driverside plastic there isn't much room for ventilation.The passengerside is alittle better since there isn't a filler neck,but still not enough room.

But still trying to hide it might be a bad thing since you need access to the plug-in,unless you run a couple different 120 outlets around the vehicle.

I will be removing bottom half of plastic and making a few shelves etc. for various things.

Larger than 700 you'll have to run large wire to the battery.At that point having an extra battery in the back might be an option.......it can get really crazy depending on how insane you start to think.

Under the passenger seat is probable enough room for a 1500watt unit with enough air flow to keep it cool.
It's about the size of a stereo amp.
You would mount it behind the front hump and have the plug-in facing the door.Pluging it in though will be rough with the seat slide in the way.
A 3 foot homemade extension cord would work perfect and allow you to tuck it under the seat from the rearof the seat.

Goodluck and post pics.

Bansil