Darrell
It sounds like your defrost drain is plugged and not draining. This is a common issue on the RF18. The drain goes from below the cooling coils (behind the plastic cover on the rear wall inside the fridge), down to a catch basin that is accessible from the rear (behind the sheet metal cover). The drain could be plugged with crud, but more commonly it is plugged with ice because the unit is not defrosting correctly. It could be a failed defrost heater or any of the other components that control defrosting. Mine was a failure of the small plastic bubble encased thermal fuse. In any case, it may take some troubleshooting to determine what the cause of failed defrosting is. Or, you can just replace the group of defrosting components (heating element, thermal fuse and temperature sensor), and hope that solves problem (cheaper than a repair call from a technician). In any case, when you put it back together, install the after market larger aluminum fin that attaches to the heater and goes down into the drain hose. It helps greatly in reducing ice plugs in the drain hose. This common issue is discussed quite a bit here on the BAC forum and on the Monacoers forum. You may want to do some research to see what you are getting into. Good luck.
Richard