I googled your model Keystone and the spec sheet states you have a 56 gal gray water tank. Unless you take very long showers, you shouldn't fill a 56 gal tank with 3 showers. I can take 6-8 GI type showers in my Class B ERA so I think either you are not getting the gray tank completely empty during a dump OR, you need to change your showering process to use LESS water.
PS---I agree with the above post. How are you determining your gray tank is FULL. I suggest you need to be highly suspicious of the tank level indicator lights as they typically are NOT accurate. Does the tank indicator show empty immediately after you empty the gray? This may be a good indicator that the readings ARE NOT accurate!