Great.....my "new to me" 2011 TT has a rotted spot in floor!

If it's near a window, I would check the seal on the window where the window frame meets the siding on the trailer. I had a couple of windows that the chalking around the frame had cracked at the top of the window. The water would follow the frame to the bottom and then run down the inside of the wall and puddle on the floor.

I resealed all of the window frames.

Also check under the trailer to make sure the membrane isn't torn or missing and allowing water to spray up