If the roof looks to not be the cause and there are no obvious issues with slide seals, etc., I would also be looking at plumbing and even the residential fridge.
A minor plumbing related leak that is not readily visible and left unchecked could cause this kind of damage over time.
My 2020 Magnitude had a couple plumbing leaks I have already addressed. One was the water line to the ice maker in the residential fridge. Another was a flexible drain from the kitchen sink to the grey tank. The other was a small leak when the sink was full of water (the drain mount was not tight enough).
Those are just some examples but there are plenty of other places like under the shower floor pan or behind the shower walls or leaking fittings behind a wall.
Before pulling the slide and getting into major work, I would be trying to disassemble what I could in the area where you think the leak is originating from and look for "internal" water leaks.
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