Ceiling Fan Installation

[Deanna A.]

Hello,

I'm trying to figure out how to install a ceiling fan into one of our bedrooms. We have a Ritzcraft manufactured home with vaulted ceilings. They seem like they're scissors trusses (from what I can see in the hole in the ceiling). There is already a light there, but just a 4" plastic electric box screwed in to the ceiling which is about 1/2" plywood and 1/2" sheetrock/plaster. I'm having a lot of trouble trying to find joists to use one of those expandable brackets for mounting ceiling fans. Does anyone have any suggestions on how to mount a fan with no support? HOw much weight would the ceiling be able to handle if there were no joist support and the fan was just screwed into the ceiling. Could we use toggle bolts?

Thanks for any assistance.