Subfloor replacement

I'm about to get busy getting my RV ready for the camping season. For several years I've been getting a soft spot just outside the shower. I was thinking it was from years of getting wet as the shower door drips a lot as it opens. After working on the check valve for the hot water heater last year, it looks like the soft spot started with a slow leak many years ago. There was a lot of work done some time in the past behind a panel under the bathroom vanity.

My question concerns the subfloor material. The rest of the front of the coach has a typical home type OSB subfloor. But at least from underneath I'm finding a black fiber type material. What might this be?