Just a little comment - if you have to replace the floor section - then in all likely hood the bolts are rusted - thus cutting a slot is fine for only as long as the bolt holds out - once it gives - and in time it will - you will now have NOTHING holding the shell to the frame or your floor
Use a cutting wheel and cut out the old bolts and remove the screws if you have any. Agree with advice given - self tapping screw heavy duty will do the trick.
Take the time to seal the new wood as well. - if moisture gets on the wood it will soak in and over time that causes wood rot. If sealed then it will wick away into the belly pan or onto your floor depending on what floor you have.
To tie the floor together we also have used butt blocks under where there was no outrigger or frame rail. Even if part of the wood made it to a frame. To give it even more strength and to tie it together I have glassed the joints on the top side.
Good luck with your repair -have you found the source of the leak that has caused the damage?? That is half the battle.