The difference between slapping 8 gauge wire 15' length from a Ford chassis main buss to the house batteries powered with a Ford 6G alternator versus running a 4AWG wire directly to same house batteries is about f i f t y amperes. Go back and note the word difference. This is from repeated experience testing not speculation.
Now times fifty amps by hours driven and raise your eyebrows. Do not make a fatal error of assumption by calculating theoretical charge curves into batteries alone. The hotel load while driving is not i sigificant.
Single Syllable Simplicity...
Eight
Gauge
Is
Too
Small