The Webasto AirTop 2000 D is a type of electrically operated diesel fueled space heater. These are found in large trucks, RVs, construction equipment and boats. They are popular in tractor trailers because the truck's cab can be heated without having to run the engine. Depending on temperature settings a unit can run 24 hours on less than a gallon of fuel.
There are two different German companies that have most of the market for this type of heater between them. Webasto is one and Eberspächer is the other. In this country Eberspächer markets it products under the name Espar. New units are pricey but there are usually some used ones for sale on ebay. These heaters are also available in a gasoline/petrol/benzine fueled version but since those are more popular in Europe they don't show up as often for sale on ebay. On this side of the pond the diesel fueled version is mostly used.