When you say diesel controller, are you talking about the Webasto plug and play controller (picture attached)? If so, they do fail. Aqua Hot has them for something North of $850. When mine failed, I got it through McCoy Freightliner in Brooks, OR, for $567 which included shipping to Florida. That was in 2015, so prices are probably much higher now.
Picture 1 - Old one
Picture 2 - New one
Click on picture to enlarge