Re-wire truck or???

[Houston Ben]

Hi all,
My wiring is not the best and im thinking it most-likely needs redone. If I turn my A/C on the volts drop a lot, if I roll my window down the volts drop, If i turn my lights on my volts drop, if my electric fan turns on my volts drop. I have relays for both fans, but they still seem to draw a lot.

So, do I need to re-wire? do I need better/thicker gauge power wires for the other wires to draw from?

What is yalls experience in this??

Thanks for any help!