Since your running from the batteries to the inverter
It's going to be almost equal length for the positive and negative lines
What has to be equal is multiple cables of the SAME POLARITY
If you double up on the wires to reduce resistance and voltage drop
You want both positive to be the same length, and both negative to be the same length, even if the negative and positive are not the same length, but in actually should be very close when doing an inverter run
Parallel connected wires sharing the load, need to be equal length, to keep the load equal on all parallel wires