There's two answers: one is a motorhome and the other is a boat - it could be argued that being a motorhome squarely puts it into "chattel" category, and it is not considered for council tax to be levied on it. One could cite the fact that motorhomes predominantly do move, quite extensively, compared to boats which often end up not moving for long periods of time.
The other answer is, there is no difference and they may seek to charge council tax on people who reside in motorhomes, which may very well have a negative knock-on effect for those with larger, and eventually smaller, motorhomes too.