That sloshing really shouldn't occur because the design of the window AC or RV roof AC uses the condensate water to splash on the condenser coil to make it more efficient. There is never enough water there to slosh while driving.since the condenser fan has a slinger ring on it to splash it on the condenser where it is force through the coil and outside.
From you saying it occurs when all is closed up I would suspect the vent is open, cabinet isn't sealed properly to prevent outside air from entering or the condensate path is restricted to the condenser coil. It is just a trough that directs the condensate to the condenser. Can't see any of this unless the AC is removed and it's cabinet is removed.
You should be able to take it to your dealer and they can make any repair or adjustment in just a couple hours while you wait.