A couple of possibilities come to mind. The drain itself could be higher than the surrounding shower floor or the trailer is not exactly level.
Take a 9" to 12" torpedo level into the shower and place over the drain and see if either of those two conditions exist. The former will be more challenging to fix than leveling the trailer. Also, there could be sag due to frame or floor issues on that side of the trailer.
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TV - 2012 Dodge 2500 4x4 Cummins HO, automatic, Centramatics, Kelderman level ride airbag suspension, bed shell
2014 31' Classic model 30 twin beds, 50 amp service, 900 watt solar system, Centramatics, Dill TPMS, disc brakes, 16" tires & wheels