Greetings R Miller,
I believe the 1800FB is a floorplan designation used by Sunny Brook, before they were absorbed by Winnebago in December 2010. Hopefully, a 1800BH owner can chime-in here with their construction material up-front, which I would guess might be the same.
Welcome to the forum.
2019 Minnie Winnie 22M on an E-450 frame