I am unclear what your options are for moving the WFCO closer to the batteries or VV to shorten the distance. Might not be a way. Whatever makes sense. ( I avoided all that by using a second higher amp charger close to the batteries and just use the converter that came with the trailer for when on shore power.)
On wire terminals, you are not limited in wire gauge by what the WFCO's will hold. Use short "pigtails" in the WFCO that will fit, and join your fat wire to the pigtail. The little pigtails do not act as chokepoints because they are so short.
Another way is to shave strands in the fat wire until they fit. I find this not so good because the shaved end is uneven in shape and is hard to keep tight in the terminal and can pull out. Maybe needs strain relief to help with that.
