While visual inspection may not guarantee safety, nevertheless it will give you adequate clues on the hygiene quality of the RV. Follow the same criteria that you will apply when seated at any restaurant i.e. all surfaces should be clean and no dust/debris.
For additional peace of mind you should wipe clean all surfaces, it will take you about 15-20 minutes to wipe clean all surface areas. Perhaps leave all windows and doors open to let fresh air in.