Hot water, Surfex HD, a handful of rice.

Fill the HT up about 1/4 of the way with Surfex/water at 10:1 is fine. Chuck in a handful or two of rice and shake like hell. It will clean up the inside quick smart. I feel I should point out that you need to remove the rice before it gets soft or you put the tank back onto the car

Surfex will clean the outside up well too. Surfex then steam would be ace i would think.