Well, if you think of electrical current as being similar to water currents, if you want more water to flow, you need a bigger hose/pipe. The wires feeding the individual circuits in your camper will be 18-14awg, and those all feed into a fuse box which has to carry the aggregate of all those flows up to the battery, so the combined current will need a 12-8awg wire. The wires between the batteries in a dual battery setup are ideally so big that that the batteries act as one bigger battery. 2awg is not unheard of. The Victron solar gear has screw terminals that will accept 6awg, so that is a good choice. The wires coming from the roof solar on a FWC/ATC are typically 12awg (I think?).
All that said, the wire size is determined by the expected current flow. Online calculators exist to help pick the right size, which depends on many factors, such as distance (length of the wire), how well cooled the wire is, voltage, current, acceptable power losses, etc.