The front windows get called into question on most rebuilds/repairs . From the accounts that I've read the windows were not the only cause of front water damage . You don't show a close up of your front window opening but from other pictures it doesn't look like there was water damage from the front windows . Personally I like to be able to look out of the camper in all four directions and to peek out the front window in the morning . If anything the side windows in the bunk are rather useless . By the time you put a mattress , pillows and blankets in it they will almost be covered . Another reason to keep all the windows is the aluminum siding is already cut out for them and to close them off would mean a patch which would take away from the originality . There is no reason that the window frames cannot be made water proof if properly installed . The operating sash is another matter and needs a careful inspection and probably fresh seals . Another option would be to make fixed sash windows for the front . Always interesting :)
