Is there some reason your toilets aren’t going to work, provided you fill the tanks by hand?
I hadn’t thought about that. What a concern would be though is if there would be enough water to do that. I’m thinking no. I live in town and am at the mercy of city services although I’m fairly self-contained if something ‘major’ happens.
Modern septic systems use pumps. They pump the excess liquid out into the drain field and the solids are what settle and have to be pumped out of the first tank every so often. We have a large system and our first tank is nearly 1300 gallons. Eventually, it will be full.
City systems should be similar. At some point, no electricity is going to mean back ups.
We always use the porta potty during floods and electric outages. Having had a major backup, it is better safe than sorry. It is a hell of a clean up job and then, you need somewhere to dispose of the contaminated clean up water.
We had an outhouse, once. We could do it again.