The ambulances I have been on do clean the stretcher after every patient, and put new sheets on. The interior does get wiped down regularly, and always after someone bled on it or threw up on it. The other things I'm not sure about. They don't sanitize the stethoscopes or the blood pressure cuff, though. Still, unless you're cut, you're unlikely to get a raging infection from that. It's still probably better, or at least on par with, things like doorknobs in any public place, or any public area that gets touched a lot and not cleaned regularly with harsh chemicals or a lot of scrubbing and soap and water.
Depends on the hospital. There is an infection control designee in every hospital responsible for insuring this is done. Good practice would dictate this be done. In the Veteran's Administration Hospitals I have been involved with they have been meticulous in this respect. On the other hand I will venture to say in any ambulance in the United States if you asked, you would find the interior ansd the cot and the ambu-bag and the endotracheal spoon and the bandage scissors were never sanitized. One shortfall in hospitals is a failure to sanitize the buttons and controls on the televisions from patient to patient, as well as the handrails on the bed.