I recently started a thread to document the radiant heating system that I am in the process of installing in my 1973 31' Airstream. For anyone that is interested you can read about it here:
I am using a tankless water heater mounted externally that can produce something in the range of 65,000 BTU of heat energy. The system is a simple open-loop radiant system where the hot water is shared between domestic hot water and hot water for heating. The pump that I pointed to above is part of the system and is responsible for move the hot water for heating around through the tubing in the floor.
I kept my system as simple as possible for economic reasons as well as for over ease of installation. See what you think...