Where did the water go???

     We just returned from the last trip of the season.  We drained out the holding tanks on our 2017 T@Bs at the campground.  We thought it might be a good idea to put a couple of gallons of clean water with a little dish soap in the black water tank to splash around during the eight hour drive home.  
     Here I am a week later getting ready to winterize the camper and when I went to drain out the soapy water, the tank was practically empty.  The trailer was tilted slightly forward so I know anything that was in there was coming out.  I suspected a leak so I put two more gallons of clean water in the tank and I was unable to find any evidence of a leak under or inside the T@B.  At am at a complete loss where that water has gone.  I'm not so concerned now because I'm putting it up but I don't want to get to a campground next season only to find out we have a major problem. Has anyone else had a similar problem.  I'm out of ideas.  Thanks.