If you can heat the house up a bit and then shut it up tight, I bet you can go a few days.
I'm guessing since you have septic, you also have a well, so no water without the generator? Otherwise I'd say leave the taps cracked so some water stays flowing. Longer to freeze that way.
You could try to drain the pipes but I'm not sure it's worth it. Will freeze in the low spots anyway.
What I would do is valve off the house from the pressure tank, so if you do get a leak, it isn't a high volume. Worst case possible would be freeze, blow a pipe, then have the power come back on and you return to a flood...