2014 Mirada Water Issue

I am having some problems with keeping water in my water tank when I go on boondocking trips. I fill up the tank before leaving and by the time I get to my destination I usually lose about 1/2 to 2/3 of my water. My trips are anywhere from 45 minutes to 3 hours. When I stop or pull over I can see water coming out of the overflow (I am assuming that is what it is)behind passenger side rear tire a little white plastic tubing. Does anyone else have this problem?

Also, does anyone know if the back window in the bedroom opens. I know it is a emergency window but I do not know if it just opens to get air???

Thanks, any answers would be great!