The front seats in a motorhome are always required to have seat belts fitted and the passengers in the front seats of the motorhome must wear the seat belts at all times while going down the road.
As for the back seats in a motorhome those seats are not required by law to have seat belts fitted and the passengers don't have to wear seat belts in the back of the motorhome.
Although you can and should have seat belts fitted and installed in the back of the motorhome seats for passenger safety if you have passengers riding back there.