I don't know your model so I am not sure where the Dinette is. Is your dinette next to a window? That might be a source. Look under the dinette seats and see if the water is coming from other sources as well. Before sealing around the outside shower door try to gain access to if from the inside, and have someone run water down the side of your camper with a hose at that location and see if it is leaking.
Model X19H Expandable
GMC Sierra Pick up
Been a camper since I was a kid, and that was a long time ago :)
From pup tents with no floors, to tents with floors and flies, to a pop up, to the expandable.