If you have a Suburban water heater, this link may help you understand the operation of...and which switches you use for either the propane and/or electric heating element.
I have a feeling you may have been not using it correct...and have always had the electric heating element on, or have never had it on and been using propane the whole time.
If it's always been on, there is a good chance the electric element has been fried, and may need to be replaced. It's all explained in the link below:
This Suburban water heater is one of the most commonly misunderstood appliances in the operation of.....especially the SWDE model that has the only switch to the electric heating element on the face of the water heater. This is why we have such a long detailed FAQ about the operation of it.
Hope this helps, good luck.