Check the top of the driver's window where the plastic molding dead ends on the top of the window. Lots of these coaches have leaked here and the water drains down to the exact spot you describe. My coach had a big gap in the sealant at this point and it was funneling water into the window frame, on down to the floor. Once I sealed this area, I had no more water on the floor to the left rear of the driver's seat.
To properly seal this area, pull the screw holding the plastic molding on and gently pry the molding off the channel. Then seal under the molding and pop it back on, sealing the outside too.
While you are up there, check for any bumps in the plastic molding, especially where it covers the joint between the front cap and the roof. Also, take the heel of your hand and push hard on the roof fiberglass directly above the plastic molding to see if you can push it in. If so, you most certainly have snapped or loose fasteners holding the joint together under the plastic mold. These coaches are known to have popped fasteners at this point which will allow a leak to develop.
Roy and Debbie
07 Diplomat 38 PDQ