They are flat with slight lateral support for the back. The back seats fold forward, in theory, and can be removed to extend the rear cargo area. RAV4's cargo area is well-designed and considerably larger than the Tracker's. It includes a moveable solid cargo cover and also has a lower load height than some utes, making it easier to slide things in the back. However, like the Tracker and a few others, the Toyota has a rear door that opens like a car's side door, instead of a hatch.
Hatches are much more practical for hauling long items.
Gas mileage was nothing special. The EPA says to expect better at 23 mpg city and 27 highway. That would be decent for a small ute. The lesson, as always, is to keep the options to a minimum if you want a small ute at a good price. RAV4 is one of the better ones.