You could put a generator on the roof, and run a cord to the inlet.

But really, get a power inverter with enough wattage to handle what you would need, and a battery with the amperage to power it. You can go dual battery or stay single, with duals and a relay, you can run the aux battery down and still start with the main.
I run Optima Yellow Top single batteries paired with 400watt inverters in my buses. Haven't had any problems starting the bus, other than after a week of camping, complete with lights and music. And then it was just a simple push start to head out.
You can wire the inverter straight into the outlet and put ina switch, to select between "shore" and battery power.