When I did my RT last year I looked at the material the AFCO gave me, studied those mostly, but also brushed up on the parts I knew I wasn't good at e.g. maths. So I'd say use the material the AFCO have provided for you, but study more specific elemnts that you know you're not strong at.
By the time you take the RT the only thing you'll be stressing over is the time limit on each section. I found that the questions themselves weren't overly complex (hardest question was gcse level perhaps?) but the time factor made the test challenging.
Each section starts with the simpler questions, and then every 5 questions or so they elevate the difficulty. Can't remember the time limits exactly but it worked out as having roughly 15 seconds per question. Also if you don't know the answer to one leave it and return to it later. If you still don't know it then just guess, it won't lose you points and you have a 25% chance of getting the right answer anyway.
