The chart in this link is an old favourite of mine. It is for sizing solar cables but works just as well for any other cables where you wish to size them to minimise voltage drop. The chart is based on the premise of restricting voltage drop to 3%. In the case of creosote 54's supply cable from crank battery to the dc to dc unit in the back of the vehicle you could use the max current of 60A (size of the CB & the total length of cable to see what the chart suggests the size of the cable cross section should be. In reality you can go smaller, because the dc to dc unit will compensate for voltage drop by increasing the voltage. (at the expense of lowering the current- amps). Minimising voltage drop in the cable will make the set up as efficient as it can be, & thus make the charging process as quick as it can be (useful for when you want to get max charge into the batteries but are not driving for long) but I wouldn't get too hung up on it, & suspect that Winnie's suggestion of 16mm2 or 25mm2 would do the job just fine.