we have a new bigcountry and have had 2 leaks in 2 different slides.
The first was the bedroom slide. The carpet got wet between the sink and bed where the slide meets the wall. Dealer said it was the sink drain which he tightened. I used paper towels placed under the sink and next to the bed to demonstrate it was not the sink, but the slide which was only wet when it rained.
While observing the slide in the rain, I noticed a production flaw that was causing our problem. The angle iron on the lower lip of one side of the slide was calked, but only down between the sidewall and angle iron. This left a depression in the caulk that allowed the rain to channel along the top of the angle iron, behind the black rubber and onto the floor of the camper. The depression was filled with caulk by the dealer and a small dam of caulk was placed just behind the first rubber seal on the top of the angle iron. This seems to have stopped the leak.
Check the caulk behind your rubber seals. We found that on most of our 4 slides the coulk stops at the rubber and does not extend on the trim behind. If any water travels along these horozontal services then it can make its way in as soon as the caulk ends