What is the best way to clean an electric fryer?
Restaurant supply stores carry degreaser, which will remove the brown baked on oil.  However, that oil isn't hurting your cooking at all.  It is the same thing that forms on cast iron skillets, but it isn't objectionable because you don't notice the color against the black of the cast iron.  If it works I'd continue to use it.

If you are thinking of changing, you might want to consider the cast iron Dutch Oven route.  They hold their heat better than consumer deep fryers and a Dutch oven can do more than just deep fry.  And they are easier to clean.