Most slides have a trim piece along the bottom sides that may need to be recaulked. If it is not sealed, water will run down sides of slide and get behind the small metal trim, than follow it inside to drop off at the inside corner of the slide. Check these out if you have them and either take them off and seal behind them or run a bead of good caulk along the top of the trim piece all the way past the side seal.
Hope this is your problem, not a hard fix.
2016 Tiffin 40 QBH
2015 38RSSA, traded
2005 TK3 #1869, 10 yrs of memories,
2017 F450 KR--one more Ford is it
2009 F450 4x4-died; 2010 F450-retired