RV Off-Grid Electrical System Wire Gauge Sizing
If you appreciate power efficiency is like rising interest rates on borrowed money, choose your tolerance level vs cost of cable to get a payback period and factor the average percent peak power demand.

For example 1% of 5.2kW = 52W in 6' of wire pair is 12' or wire. That's about the same as waste heat in a 60W light bulb. 

But that rarely happens, however when the ACU starts it draws up to 5x the current and if the voltage drops, it takes longer to get up to speed which is hard on the motor.  Generally, a 10% voltage drop is the spec but the source impedance is much lower. 

Due to motor/compressor start surge currents on DC you want a &lt;1% voltage drop to minimize loss of motor torque during start.

The Tesla 5.2kWh battery is 24V, 233Ah, 5.2kWh with 23.1V @ 70% SoC and 
24.9V @ 100% SoC.  ( it is not 12V nor should label say +24,-24 rather 24+,0V )

Thus assuming a 3kW load draws 5x or 15kW surge momentarily or 15kW/24V = 625A 

With 6' of wire pair or 12' total  with a 1% drop in voltage  0.24V/625A=0.4mΩ

This would be slightly bigger than the  size cable used for a car starter or 5/0 welding cable.

But you can get away with 4/0 Welding Cable which depending on your supply of surplus auto cable or new 15' will be much less than  $10/'

Connection torque is important just like on a batttery.