Espar and Webasto make super diesel furnaces. I liked the Webasto Airtop series better and hance replaced the rusted Espar with a Airtop 5000.
If you are considering a Webasto, be sure to consider sure marine in WA. Their manual is amazing, and considers just about everything... evidently the product of a lot of time, experience, and customer support calls.
Construction quality and general principles of the Espar and the Webasto are similar. However, the Webasto cost me a bit less.