A few observations: "Hot water tank"? You can't actually "see" the level in the water heater. Thus you must mean the fresh water tank.
If the tank level is dropping even when connected to city water your drain valve must be open or partially open (even when fully open the fresh tank takes forever to drain so it could be fully open). Look on the floor near the tank for a valve; the valve's handle should be perpendicular to the pipe (off). Also look under the coach if the valve is open you should see water dripping onto the ground.