If the tank is empty then the problem could be a plugged P-trap or blockage anywhere between the tub and tank. A venting issue for that holding tank will also create drainage problems. If the vent stack has slipped deeper into the tank (below the level of the contents), the tub will likely back up or drain very slowly. But the fact you have water leaking tells me you probably have a full holding tank and/or a failed termination valve (dump valve), and it simply backed up into the tub.
If, by chance, the shower drains into the black holding tank, shared with the toilet, and you only evacuated the gray tank, check to make sure both holding tanks are completely empty and see if the problem persists. Also, with both holding tanks empty you can easily find out which tank the shower drains into by leaving the termination valve open on one tank, then run some water through the tub and see which tank it drains through. It is permissible, by the way, to have another fixture drain into the black tank and many floor plans mandate this unfortunate feature.
It's also recommended to upgrade your termination valves to electric gate valves. Check out the Drainmaster electric valves.