If you want to monitor multiple tanks, you can have a look at this project

https://helloworld.co.in/demo/water-tank-widget/   

But if you want to scale this project for 5 tanks, you are almost correct. you need to have 05 python scripts running on RPi through 05 cron tasks, each recording data from respective sensor and sending to the common database table. Just add a column, say sensorID, in level_log table to identify from which python script the data is coming.