Yep, those drain slots need to be clean but:
There have been a few of us that have had a window leak because the butyl tape used to seal the windows to the fiberglas did not properly bond during the mfg process. This is not just applicable to BF's.
Look at the top of all of your windows and see if there is a crack between the tape and the fiberglas or the window. If you see gaps it is time to think about removing the window and re-sealing.
When I got my BF, new back in 2000 my kitchen rear window leaked like a sieve! The mfg did not put enough tape on it to seal around the slight curve of the rear of the camper where the window is... I fixed it myself... Then I went around and looked at all my windows... The windows were barely seated into the tape. I went around each window and gave the screws about 1/4 turn. Some were so loose that it took a whole turn. This did cause the tape to ooze the next summer. I trimmed the tape 2 times over the years. My windows are really sealed nice, even after 18 years.
Jim