Using 4awg for nearly all of the run and then reducing to 6 or 8 awg for the last few inches, assuming good connections, would have lower overall resistance than using the smaller wire over the whole length.
In this case, I may be able to provide information that can convince the OP that 6 awg is adequate. I have a 2021 GMC 3500 hd crew cab long bed with a Grandby front dinette, which has its battery compartment at the left rear of the camper. I suspect my wire run is about as long as any truck camper would have, and I can look closely at it to estimate the total length later today (it’s about 30 feet one way). The truck has a smart alternator obviously. I use the Victron 12/12-30 isolated Orion Smart charger installed at the rearmost part of the camper battery compartment.
With about 60 total feet of 6 awg Ancor Marine type 3 wire and an SB50 connector at the front of the camper, the voltage drop is low enough that the Orion’s engine running verification cycling function is NOT triggered at any time. The charger’s engine running detection function is able to detect engine running purely through the basic voltage detection thresholds. This is what you ideally want with the Orion to avoid cycling and lower average charging output. I would suggest reading the engine running detection portion of the Orion manual if what I’m describing is not clear.
I can make a more precise estimate of my wire run length and answer other questions about the installation for the OP if desired.
