[puddleglum]

It's good you have some protection already. You could let some reflective insulation in the existing under tray and some over top of the engine. I think it would help and be cheap and easy. As for your battery calculations, in theory you are right about the AH and amps required, but nothing is 100% efficient. Batteries are rated at a 20 hour rate. The 1 hour rate is about half that. Also, you don't want to discharge below half capacity or you will get very few cycles out of it. Do you already have a block heater in the car or do you have to buy one? The inverter will have a percentage of losses as well.