For those Aqua-Hot owners who haven't read your owners manual, it clearly states early on in the manual that the Diesel Webasto Burner is your PRIMARY heat source and your Electric Assist is your SECONDARY heat source.
The 600D model with two high wattage electric heating elements may keep up with demand during cold weather depending on the temperature outside and the temperature of the incoming shore water. However, even that manual clearly states that the Diesel Burner is the primary and the electric assist is the secondary.
Also, the recommended procedure is to ALWAYS run your diesel burner at least ONCE per month especially when you have been using ONLY the electric assist. This is to ensure that the diesel system stays in good working condition.