Do you keep your slides in or out when you are not using it? If it is out, I've always double checked all rubber slide gaskets to make sure they are fully out making a good seal. My dining room table slide I notice sometimes has 90% of the gasket out and the bottom 10% still folded in not making a solid seal. Water will make its way anywhere it can get to so a little section of a poor seal will travel wherever gravity can bring it.
Just one thought for you to check if you keep your slides out while not using it. If not, I can't help any more then that lol.
2015 F350 6.7 Platinum
2015 Coachmen 360IBL