Have you used it since you drained it? No real basis for my theory, but a loud thud may sound like a broken inlet pipe from heavy weight on the tank causing it to sag and seperate from the inlet tube. If you have used it since it was drained, then my theory would be all wet (pun intended). Hope that all is ok and no damage done. I think I would cautiously run water into the grey tank from the sink and see if there is any leak around the top of the tank.
Lee and Diane
2008 Jayco FS36 bunk model/entertaiment center
2009 Jeep Patriat toad
Retired and enjoying new grandson