I have a 2013 C Max with essentially the same drive train. Do not know enough about battery replacement but it is many thousands of dollars. I have read about the topic and do know that the prior generation of batteries last a very long time. The new Ford hybrids use lithium batteries that are supposed to last longer.
The Fusion hybrid should be the same as the 2013 model and my C Max, I.e., towable four down. Mine tows VERY easily. Just remember you must install a charge line between the RV and car. Without the charge line the battery will discharge quickly and you will not be able to start the car after a couple of hours. The car's ignition must be in the "ACC" position and transmission in the "N" position.