Not familiar with your specific rig, but when I would over fill my fresh tank on my previous coach, the fill line would create some siphon action and drain out the fill line and the overflow until it dropped the level enough to break the siphon. ( Should have had a check valve, it did not). It could also be the fill valve is stuck and allowing water to continue to fill the fresh tank. Is your overflow also dumping water? What is the water level on the tank itself?