This is not an uncommon problem. WD-40 is not the product to fix this. If your gray tank valve is accessible, you need to remove it, check for binding spots especially around the blade. Disassemble and lube with silicon grease. If your unit has one of the long cable runs, you will have lots more fun. Check the cable run for sharp turns and binding points. You will probably need to pull the cable and lube it as well. The leak may or may not be a separate issue with debris.
2016 Winnebago Journey 40R - 2012 Jeep Rubicon
2 Arctic Cats, 3 Shelties, 4 cats
Bottle of Jack Daniels & wife