I'm in the process of fitting side steps to mine and the rivnuts supplied are 10mm.
They fit nicely into the pre-drilled holes in the chassis but getting them to 'crush' up properly is virtually impossible as the hole, particularly the top hole, is close to the rear side of the sill.
To stop them pulling out I first smeared them with liquid metal,
EVO-STIK Serious Stuff Liquid Metal/Wood/Glass/Car Ultimate Grab Adhesive 290ml | eBay inserted them and crushed them up as tight as I could (using the method given in the instruction leaflet).
I then bonded a thin 10mm nut on top of it, making sure the liquid metal gave a good surrounding of each nut. (I kept the bolt in place for this to ensure the nut stayed in line).
Hopefully this will ensure the rivnuts don't pull out as, previously mentioned, every time I tried to get the rivnuts tight enough they never crushed up inside the chassis properly.