I having been through this all before, and with multiple tanks with some very pricey breeders and fry.
I. Remove any biomedia from external filters and place it in the tank.
2. Oxygen and circulation can be maintained by using battery powered air pumps.
2 a. Peroxide can be used to supply O, but be sure to research dosing/use issues first.
3. Stop feeding. Most fish can easily go a week w/o food.
4. Water temps can be maintained a few ways. I have removed water from tanks into pots and heated it on my gas grill and returned it to tanks to maintain higher temps.Heat retention can be increased by covering tanks with towels, blankets etc. to keep in heat. Ice, placed in bags, can be used to cool tanks.
The best thing is to have a back up generator. I am fortunate now to have a whole house backup generator system. The longest our power is ever out is 20 seconds. However, a smaller gas powered generator should be sufficient to power filters and heaters in a number of tanks.