Why a analog INA169 ? Is that because the distance is too far for the I2C bus or is there no 60V I2C version ?
You can step down to 5V and power a Arduino via its 5V pin. There is a risk that current can flow into the USB port of a computer.
A problem could be the GND.
With high currents through ground wires, a good common ground is hard to make. If you do something wrong than many amps might try to go through the Arduino board.
When the ground current can take two routes, the ground current can decide to use a smaller wire and ignore a strong thick wire. For example when the small wire is inductive coupled with the positive power wire. For higher frequencies it is often worse. Current spikes are higher frequencies. You have indeed high frequencies in your 12V DC system.
I would add a few good old fuses, so a shortcut near the Arduino will not damage the rest.
How do you display the numbers ? A LCD display is not durable in a camper (sun, heat, vibrations).