Hey Keith, we had the same issue a couple of weeks ago. CAMCO make a drain valve lube that prevents sticking drain valves. Camco are the folks that sale the treatment for your gray and black water holding and waste tanks. You can find this in Camping World and online, it comes in a 16oz plastic bottle and is very easy to use. Basically drain the black tank and flush it out good, then add 4oz of drain valve lube along with 4 gallons of water. Next what you want to do is open and close the valve in short succession several times to get the lube to penetrate the blade valve on the tank. We had to repeat this 3 times in order to get the drain valve sliding smooth again; it work for us and I'm sure you will have succes with it.