Ok
1) a battery generator won’t run 1000 w heaters for long. The ratings are different
Example - - 2000 watt hour battery generator has a 15% loss due to efficiency so the running time for a 2000 watt battery powered generator is
Working time = Wh* 0.85 / operating power of your device
(2000 * .85 ) / 1000w heater = 1.7 hours
2) you can prep for this. Every hour without heat your temp drops closer to ambient. What’s your current temp? Can you afford to raise by 1 degree temporarily for a few days. It could buy you a few hours
3) buy a oil filled space heater. They look like a radiator from the 60s and pump out massive heat. Pump that room ambient temp up as much as you can the day before the outage
Insulate the tank before you leave. Packing blankets, Mylar wrap. Whatever.
6 hours will not be long I would only expect the tank to drop a few degrees with some careful planning