I change mine every 30,000 to 45,000 miles. The book says every 10,000, but that assumes the dirty and low cetane fuel we have here in North America as compared to that in Europe. If you don't drive much, as with an RV or mostly in town driving, and don't use an additive, then changing it out at 10,000 miles is probably a good idea. But when you're filling the tank several times a week and racking up the miles, and using an additive every tank (or nearly every tank), then the fuel in the tank isn't going to pick up contaminants the same way that stagnant fuel in a tank will, then extending the change interval is not a problem.
Using a good additive like Howes or Power Service does more than clean the injectors and keep them clean, and boost the cetane in the case of Power Service, they also keep the tank itself clean, and thus the fuel in the tank is cleaner. To illustrate that point, pull and inspect the in-line fuel filter on your Espar heater. It's just a fine metal mesh cup about the size of a pencil eraser. If it's clogged and dirty then you haven't been diligent with your additive. If it's pristine clean, there ya go. It's an excellent gauge as to the condition of your injectors.