Every installation guide I've seen includes placing a sheet steel floor where the generator is going either for fire protection purposes or to ensure the dynamic loading of the generator is not too much for the floor. I suppose it's up to you if you want to do it but, personally, I would.
I have not done this installation so I cannot help with the gauge of metal to use. If I were to do such an installation, I'd probably contact Crossroads or Onan and ask them for some information to assist me.