OK, couple of things here --
1st - NEVER leave your RV for a few days with water hoked to it.... If you had a leak it could have been catastrophic..
When I leave my RV un attended for a period of time I do the following: unhook the water line and shut off the water.. shut off the water heater... CLOSE all tanks.. put slides in ( optional in my eyes )
2nd - was/is there actually 2/3 of water in the tank? Or are you going off the gauge.. If your going off the gauge, they are wrong99% of the time
IF you are telling us that you have no standing water, no dripping water, I would assume that the gauges are just covered with slime