The maximum speed limit.
It is the same as posted passenger vehicles. Trucks/Cars with trailers have a posted speed lower than passenger vehicles so if the motorhome is hauling a tow car then they should follow the posted speed limit for vehicles with trailers.
the same speed
No. Upstream of Teddington, the Thames speed limit is 8kph (approx. 5mph). The rest of the river has no speed limit.
same as driver
Unfortunately, you can not limit speed for guest networks. They will have the same access to the speed available as you do.
Speed limits are based on the road, not the driver. Thus, unless another limit is specified, such as for trucks over a certain weight, the same speed limit applies to all drivers on that road.
The speed of light is always the same as long as it's traveling through the same medium. But its speed is different in different media, and those are all less than its speed in vacuum.
The same as it is for any other vehicle.