How is it OK to use Truma Heating without water?

I understand that Truma Heating can be used with an empty water tank, but how does that work and can it be done for long periods of time. There is little info. regarding this. We would like to use our motorhome for day trips during the winter and don't want the problem of filling up and draining the tank everytime we take it out. But would like to be warm.