Is it normal to fill up the grey water tank this fast?

Alternatives-
1. back flush your gray tank to make sure there isn't a partial blockage.
2. Drain the gray completely. Using 1 or 5 gal. bottles of water and fill the tank to see how many gallons it takes to indicate tank is full.
3. check your mannual or call the mfg. to verify the size of the gray tank.
4. Use paper plates
5. occasionally empty some gray water into the black tank.
6. buy a "blue tank" to empty the gray tank into when it indicates it is full.