[JRTJH]

Tbos is correct. The slits are there to allow any condensation that occurs on the inside of the window to weep out from the inside bottom edge of the window frame. Also, if you notice, the frames are not insulated, so in very cold weather you may find some frost, even ice along the lower edge of the window (in the bottom lip). Those slits, again, allow that water to weep out of the window "ledge" once it warms up and thaws.

They are normal, and on almost every RV window, so leave them alone and just keep them clean.