I like the idea of a very convenient metal storage box and your neat thought of having it on the door with the weight on the bumper. Three hundred pounds is scary, though.
I have a nice little 1000 watt generator for power outages at home - we live fairly far north in Canada and loss of our furnace on a winter day would be a serious matter. But I have never thought of taking it camping. For 20 years of camping with a popup trailer we charged our battery (a portable booster) from the van's alternator system. Now we have a 100 watt solar panel (system including battery controller and monitor cost less than the generator) that seems to keep the batteries properly charged for as long as we like. The weight of the panel is far less than a generator, it is out of the way on the roof, and it reliably and silently provides all the power we need.