Moses shows no other symptoms of anemia other that the white gums. She is very active with our other Doberman, and her eating habits are normal. There are a number of possible causes of anemia in this situation. I am a little uncertain about the meaning of a 'negative' von Willebrands test. I am going to go on the assumption that is the case in the rest of the answer.
Anemia occurs for a number or reasons but they break down into two major categories.
If the destruction outpaces the production, anemia occurs. Kidney failure, bone marrow disorders, some cancers, some toxins, inherited disorders, ehrlichiosis, drug reactions and iron deficiency can all lead to non-regenerative anemias. Even severe flea infestation can cause significant anemia in young dogs. There are a lot of possible problems. A general blood panel and a clotting factor profile are good first steps in figuring out what is going on.