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.
Eagle5
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2019 Minnie Winnie 22M on an E-450 frame
According to the specs online I see Aluminum framing with "fiberglass" sides. I'd bet that's fiberglass reinforced plastic (FRP). On current day TTs the sidewalls are covered with Filon brand FRP.
That's just a guess... and who knows if the online info is correct, too.
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2017 Winnebago Adventurer 37F
2016 Lincoln MKX Toad