See installation instructions for the Atwood charger.
APS MODELS: 32, 45 AND 55 AMP
APS-3230, APS-4530 AND APS-5530 - 30 AMPS AC MAIN
APS-5550 - 50 AMPS AC MAIN
Atwood made a 30 amp 120 volt AC model and a 50 amp 240 volt AC model.
If you have a 30 amp model you need 10 gauge 3 wire cable for shore power.
The 50 amp model requires 8 gauge 4 wire cable.
The 12 volt DC side of the converter charger can be 32, 45, or 55 amps.
Assuming you have a 55 amp charger, you need 8 gauge wire from the Atwood to battery, both positive and negative. You also probably need a 55 amp fuse in the positive wire.
4 gauge wire would charge a little faster if you have a large battery bank. I am not sure if you said you have three 12 volt batteries. You could use a 100 amp fuse with 4 gauge wire.
I wish you good luck and happy trails ahead!