We have a 2018, 3100 also and we have not had any problems with water getting inside with the slide open or closed. Have you checked the top of your slide before you close it to see if water has gotten on top of it while it is open? We have found that the slide awning topper that PC uses on the 3100 is barely long enough to cover the top of the slide and if there is any wind blowing when it is raining, water can accumulate on top of the slide. Also, when it is raining, a small amount of water can accumulate on the awning topper itself and when the slide is brought in, it is possible for some of that water to run off the end of the slide awning topper and get on the slide's top or end panels.
As a result of experiencing similar issues on an earlier RV, we purchased a collapsible ladder and an extendable handled squeegee that we use to remove any water that has accumulated on the top of the slide awning topper AND/OR on top of the slide before we close the slide on our PC. It is just one of our steps in our closing procedures that we check if we have had rain while we have the slide open. An option for you to try if you are unable to check the slide's top before closing is to have towels ready when you close the slide and lay them on the top of the slide to absorb any water that may have accumulated on the top of the slide as you bring it in.
The only way to prevent water from accumulating on the top of the slide is to replace the slide awning topper with one that it approximately 6" longer that will hang over each end 3" which would prevent water from getting on the slide's top. We did this on an earlier RV and the problem was resolved.
Hope this idea helps.
Gary
