Generally if you have tanks with Firly fine (Small) wires coming out of 'em those are the level sensors.. And on waste tanks (The black plastic tanks are usually waste) they really are useless after first use so don't worry.
But they go to an epoxy block called a resistor pack From there one wire goes to your sensor panel. Each tank has its own resistor pack and lead to the sensor panel. ONE sensor lead (usually the black and it is the BOTTOM one) will go to ground. Usually a ground pad on the sensor panel
And as I said. they basically work.. ONCE.. Now if you have SEE LEVEL sensors (After market produt) That works Period. but the wireing is different.