Can I plant a new rhododendron in a location where the previous one developed fungus from overwatering?
The spores are in the soil. It would not be the best plan to replant another rhododendron without first taking a soil sample for analysis by the Penn State Cooperative Extension Service. Penn State will have such agents which you can find if you Google Penn State Cooperative Extension Service and find out how to take a soil sample and where to send or bring it and what information you are to list in order to receive the facts you need.
Some regions have plant nurseries who can sell you the soil collection envelope and will know where to send it.