You would do best with a UPS. Around $120 and you can keep filters alive for quite awhile. Add backup batteries and you, without question, will have the filters running for days. Play it right and you can exchange batteries every day or so and charge off of your truck.
The generator is also certainly viable. Making your own power after a tornado is certainly worthy of a premium price.
Also, I don't know exactly what you ask out of a contingency plan, but to me, it is absolute essentials. No lights, no heat, no auxiliary devices. Just the filter and "enough" aeration to keep the fish alive.
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