1. If you're referring to the battery line that feeds the unit, 10G will be fine in general, it really depends on the length of your run. There's a chart below, 10G is good up to 10A at 28', so unless your converter/distribution center is on the rear bumper, you're fine.
2. While truly equal leads are best, the entire system doesn't need it. You want equal length leads between the batteries so that they stay balanced. The ground is going to tie into the frame fairly close to the batteries, the positive is going to run to wherever the distribution center is. After the batteries equal lengths aren't as critical.
3. Unless you're going to A; run off one battery, then the other or B; leave one battery at home to save weight, yes. Just tie them together and leave them. That way they're always ready to go, you won't have one that got low dragging on one that was in good shape.
You can disconnect either lead to relieve parasitic draw, most folks put the switch on the positive side but if it's easier to cut the negative side then you can place it there. Just place it between the cable actually going to the camper. One click, or remove one nut, and you're golden.