Here is a link to one of those on Amazon:
Your RV battery
and an inverter do exactly the same, for a heck of a lot less money.
These things should not be called "generators", they are simply a battery
packaged with an inverter. A generator
is something that converts mechanical energy to electrical
energy, without storing it. That mechanical energy comes often from gasoline via internal combustion engine.
The battery in this device must be charged from a real generator, be it one that you bring along camping, or one that your trusty power company runs to give you your home electricity. It came from coal, gas, nuclear, hydro, solar
, or such, and ultimately from the thermonuclear reaction in the Sun. Also, with every conversion, transformation or inversion and transmission, you get less and less out, a fraction is always lost in waste heat.
So, I wish the makers would actually tell people what it is. It smacks of false advertising, and if people believe there is something to it, I have a bridge to sell you.