I had a 94 Dutchmen 5th wheel and all the windows had a screw thru the plastic outer covering in each corner. I r&r'ed all the windows replacing the gasketing with foam weatherstripping. If I remember correctly there are screws under the plastic and inside the window frame that clamps the inner and outer together. It has been 6 years so the exact details are a bit foggy but it was not a bad job. Two of the windows were 4'x3' and the rest were much smaller, 5 in all.
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