I normally say "upgrade it all the way" but JonR has a good point. So, if your TOTAL run length is 50' return, then changing the internal wire (not more than 10' worth I would suspect) will result in only 80% of the benefit of upgrading it all. Is that enough?
Using a voltage drop calculator, and assuming your alternator is putting out 15V (that would be very high) I see 31% voltage drop (4.66V) on 14awg. If that's the case, then you will only see a bit less than 10V at the batteries. (15-5=10). Obviously not enough to charge your house/camper batteries.
Just guessing, preventing 80% of that loss might be enough... 10% of 4.66= only about 1/2V drop, = 15-1= 14.
But wait... there is still voltage loss even over 4AWG. 0.37 volts to be exact if we use 40' of 4AWG (ends at the Anderson Power Pole connector).
Plus the loss over 10' (20' return) over the internal 14AWG = 0.93V. Combined voltage loss = 1.3V. leaving 13.7V.
What is the recommended charging voltage for your AGMs? I think 13.7 is too low for most AGM's (never mind LiFePo4 !!!)
BU meta description needed...
batteryuniversity.com
The way around this is with fat wires all the way and/or a DCDC charger.