You could walk around in a motorhome while the motorhome is going down the road but it can be hard to walk around in a moving vehicle.
For safety you really should always remain seated while the motorhome is moving unless you really need to get up and go into the bathroom to use the toilet.
When you're standing up in a motorhome walking around it gets unsafe because if the driver of the motorhome had to suddenly stop then you'd be thrown around and may even be throw into the windshield or other part of the motorhome which could cause serious bodily harm or even death.