I'm guessing you filled the on board water tank completely full?
If you did, or really close to full, the water in the tank could start sloshing as you drive down the road, and empty itself using the overflow pipe. It siphons itself empty.....very common.
To test the theory, fill it when you're parked, but not so full as to have it start emptying itself. You shouldn't lose any water. If you only lose it when you drive down the road, install an inline valve to close the overflow. Remember to open the valve before using the onboard water or you could collapse the tank sidewalls.
Worked for me.
Ken and Terry
2010 Georgetown 330TS being pushed by our
2017 GMC Terrain toad (4 wheels down)