Webasto and Eberspächer (espar in americas) make RV diesel heaters that heat both air and water. If I only remembered the model names just now 
But they do exist.
Planar is diesel air heater (russian copy of webasto) and Binar is the water heating copy of webasto then, I believe…
Also, if you are searching marine parts, look for Wallas diesel cooktops. Some of the old models can be retrofitted with new top cover that includes a fan. It allows the cooktop to become a diesel air heater.