In addition, you would need to enable and rename the same Attrib_ field under Maintenance / Data Field Definitions / Site fields (e.g. rename it to "Allow Trailer"), so you can select which sites allow them. Then go through Site Details for each site, under More Attributes, and set that to Yes for each site that does allow trailers.
Now you can select that Attribute on New Reservation when needed to show only sites that allow trailers.