A new fifth wheel hitch package would have come with 6 or 8 heavy angle brackets ( 1/2" thick ) to attach the through the bed floor bolts to the frame. If your hitch was installed without these brackets, don't tow with it until they are replaced, because the P/U bed is attached to the frame with only 5/16" bolts , no where near what's required for safe operation.
The hitch package would have had 1/2" bolts ( 3/4" wrench size ) every where, bracket to frame , and bracket to hitch rails.
Visit a hitch shop, that does the installs properly ; yours wasn't ; and see if they have extra angle brackets.
Visit the hitch manufacturers web site for proper installation instructions, and a picture of what those brackets should look like.
EDIT: Some trucks manufacturers are very particular about drilling the frame rails for install , so check the truck makers web site too.