Sounds like you have some aileron and incidence issues, try banding some straight sticks under each wing panel at the aileron inner cutout position and then eyeballing from the side, that will very quickly show any differences. It also sounds like you are trying to fly it like a normal model airplane, and learning that it isn't. Full size Camels were always launched straight int the wind, anything else won't work. The rudder is your main directional control, on the ground and in the air. Use full up elevator at the start of the ground roll, this keeps the skid on the ground and the drag behind the wheels at slow speeds, helping it to stay straight. Release as the rudder bites and steer with rudder for the rest of the flight. Ailerons are there to help balance the rudder induced roll. Use the throttle carefully, rapid movements will unsettle the model. Get as much mechanical movement on rudder and elevator as you can, rudder is your directional control, and you need lots, as you have found out. Elevator is your speed control, and you also need lots of 'up' on the ground to keep the prop tips out of the dirt. If you find that you are still having problems educating your left thumb the you can try this little wheeze; put the axle through a couple of tube bearings where the bunjees are, and tie the wheels to the axle so that the wheels and axle rotate all together, this will help to keep it straight so long as both wheels are on the ground. You won't be able to taxi the thing any more, but then the real one never indulged in much taxiing anyway.
Evan, WB #12.