The two previous Class B RVs I had sat 5 with seat belts. The 2005 Pleasure-way Plateau and 2011 Great West Van Legend had electric sofa beds with three seat belts. The side had shoulder belts and the middle only had a lap belt. We used the middle when our grandchildren still traveled in a baby seat with its own shoulder harness. Neither had the headrests on the sofa seat and that was a concern.
We took our adult son and two grandsons (6 and 2) on a trip from Wisconsin to Virginia and back of about 1,000 miles each way. They slept in a tent. It would have been a lot easier in a passenger car.
In my younger years we traveled in station wagons but that was in the days when you put the seats down, threw the blankets and pillows on top and traveled loose. Somehow our three kids survived.
I personally think a B is maxed out for two people. Beyond that you have little room for necessary luggage and personal items. The on board facilities and tank capacities are not designed for additional people. The most capacity for sleeping inside a B might be the Roadtreks with sleeping for 4 and I think that would be with two pre-teens at best. Then at night the Roadtrek B is all bed like a tent.
Consider something like a passenger van set up with some amenities like an on board toilet and kitchenette but carry a tent and travel like a tent camper. Midwest Automotive Designs has a DayCruiser model with a bath and kitchenette and will carry 6 in captain chairs facing forward. Here are some photos.
https://www.flickr.com/photos/davydd/se ... 936738211/
Other than that you really should be looking at Class Cs, trailers and bigger.