Per a voltage drop calculator using 6 awg wire, 40 amp load (though your charger is 75 amps), 12 foot run:
Voltage drop: 0.38 volts
Voltage drop percentage: 3.17%
Voltage at the end: 11.62
Going from 12 volt to 11.62 doesn't sound like much, but I feel the drop will greatly increase your charge times in bulk charge mode of 14.5 to almost 14.1 volts.
My camper's converter was 25 feet rear of the batteries up front. That distance and thin wire caused a .5 volt drop which I didn't realize or consider due to lack of knowledge on the subject. I couldn't figure out why it was taking me 8 hours of running the generator to charge my battery banks each day after running heater all night. I had upgraded my converter to a 70 amp PD 3 rate charger and was upset that it wasn't charging much faster than the stock 40 amp charger. Moving the converter within 3 feet of the batteries fixed the issue. Morning charge is about 2.5 hours on generator but since then I've gotten solar and the generator is collecting dust.
You should heed the words here Johnm1, He has it right.