If you fresh water vent tube comes out from the bottom of your unit, then your water is being siphoned out/ sloshed out through that. Very common problem now a days. Put a shut off valve on the vent tube to keep the water in and when set up, open it up and blow (yes blow) air back up through it to open the vent up and get the water out of the low point in the tubing. Major engineering screw up.
2013 Forest River Salem Hemisphere SBT312QBUD
2001 CrewCab F-250 7.3 PowerStroke Diesel
SuperChip, BTS transmission, 6.0 Trans Cooler