I have never seen, nor heard of a float switch in a septic tank
And don't even think about using the same relay to switch 12 volt and 120 volt circuits.
Two strikes ...
Float switches are very common in marine sewage systems as well as shoreside sewage transfer systems.
What? Are you worried that some mysterious event will cause a crossover or something? That is no more likely than adjacent contacts shorting out within the relay case. If we were to take your position we would use separate relays for each phase or line in the circuit and all relays would be single pole single throw.