I don't know of a drain tube available. We too get the water run-off from the A/C. The rain gutter diverts it rearward and then out, but it's a hit or miss if it drips down to the ground or down the side of the RV. It depends on the angle of the parking pad.
You might want to try a plain old wooden pinch type clothes pin attached to the rain gutter at the approx drip point. The water will go to the end of the clothes pin and drop off instead of running down your paint job.