Why is my voltage drop so awful?

[osl 10+ year member]

I felt the same way when I had Voltage problems a while back. You will be back.
You need to run two runs of 1/0 awg from your front batt. to rear batt.

You need one positive run and one neg. run from front batt to rear batt.

You need to wire your amp directly to the rear batt postive and negative.

Make sure your alt is upgraded 1/0 awg to the front batt. as well as 1/0 awg from negative alt. (case of alt.) to negative of battery.

Just take your rusty truck frame out of the equation all together.

I think 2 runs is a bit extreme for that amount of power. I'm guessing that you need a new alt or maybe even the alt belt is slipping. That and like said a million times before, check your grounds and make sure they are tight and on bare metal.