I replaced the front bunk on our previous 2011 Roo 23SS. The rubber flap you are describing was secured inside the front cabinet under the bunk. If you look inside the cabinet, there should be a 1x3 piece of spruce that is screwed to the front wall with a couple of wire ties securing the marker lights. If you remove the wire ties, and then unscrew the 1x3 you will find that the rubber flap is secured under the 1x3 with a few staples. You will need to fold the edges on a 45 when reinstalling the new one to prevent the flap from binding when the bunk door is closed.
I replaced the entire bunk door and hinge, located the cable supports and locks in a matter of a few hours. It is a pretty easy project for someone with basic skills - just take your time and triple check your measurements before drilling for hardware on new door.
2014 Rockwood Ultralight 8312SS Diamond Edition
2011 F150 Super Crew 4x4 5.0L - Traded!
2016 F350 Lariat Ultimate Super Duty Crew Cab 4X4
Northern NJ - God Bless America!