As Roger explained, in the
Bigfoot line only the 17.5', 21', and 25' trailers are moulded
fiberglass - they are the "2500" series, and none have slideouts.
The other
Bigfoot trailers with aluminum frame and sandwich panel construction are the "3000" series - they do have slideouts available. I suspect that
Bigfoot introduced the new line, deviating from their traditional practice of moulded construction, at least partly
because they wanted slideouts.
I find that the "series" number is the key to sorting out Bigfoot products by construction, both in discussion and in their web site.