New to Us KZ 260

This would be the '07 KZ 260RBS you asked about recently here? So it has a decade of unknown treatment, mileage and maintenance? This is where it's always a good idea to hire an RV tech beforehand for a thorough inspection, even on a new TT (ask me how I know).

We have a '14 KZ 262RKS also with a long-ish slideout. I do not know how they are put together because it's covered by Darco underneath and the solid floor above. Our slideout has heavy components in it - fridge, wardrobe & aftermarket power recline loveseat (rather heavy). Has not been a problem weight-wise over 4 seasons and about 20K miles so far. The slide floor has open cell insulation in it which will absorb moisture.

I drove for about 3 hours in a torrential rainstorm through WA last fall. When I got home, I found some water on the floor near the slide. Have not investigated it yet but it could be getting in under the slide from water being thrown up by the wheels. There are some spots where water could get in and could be getting wicked in via a crack somewhere or maybe the slide gasket. Will look closely in better weather this spring or summer.

It is possible you have a rotten subfloor. While our main floor is plywood, the slide floor is OSB. If the edges aren't treated after cutting it, water can get absorbed and it can swell, soften and disintegrate. This is not uncommon and have seen posts about it on this forum. It involves some extensive reconstructive surgery to fix it. Leaking slide gaskets are common cause. Assuming there is carpet on the slide floor, I'd peel it back to see if there is anything unusual looking.

Water intrusion is enemy no.1 when it comes to RVs. If that's the cause, besides rot there can also be mold in there so be careful. Not sure if the idea of wheels is a good one but you first need to know exactly what's going on before you undertake repairs.