In Oregon (where I live), passengers of motorhomes are required to occupy any of the FORWARD FACING seat belts as long as a seat belt is available. If there is not one available, you would not be breaking the law to have additional passengers that do not have a seat belt available. Let's take my 2008 Winnebago view as an example. It has:
2 forward facing seat belts (driver and passenger front seats)
3 side-facing seat belts on the couch
2 rear facing seat belts on the dinette.
Therefore, as I understand it, with my family of 4, each of who are over age 8 and over 4 feet 9 inches, according to Oregon law, only 2 persons need to be in seat belts.
That said, as a practice, we still keep all of us in seat belts while we are traveling, even though my perception is that the rear facing lap belts on the cheesy press board dinette seat back and the side facing belts on the couch are likely only good enough to find the bodies in a roll over crash, but are better than nothing in most other accidents, but there most certainly will be injuries.
As an aside, I contacted Winnebago about the option to upgrade our dinette to the dinette setup that is in a Winnebago Trend (steel construction, high back seat, 3 point harness). The trend has to comply to more stringent crash standards due to its lower weight causing it to meet certain crash standards. As expected, they give no recommendation.
Exact quote from Winnebago:
We have not performed this modification so we would not have any specific recommendations, nor could we guarantee the results.
I hope this information is helpful. If you have any further questions, feel free to contact us at 1-800-537-1885.
Oregon Law states as follows:
Motor homes are considered passenger vehicles under Oregon law and as such, adult belt and child seat requirements apply also to motor homes --- but only to forward-facing vehicle seating positions
(those meeting federal safety standards for seat belt anchorages). Therefore, occupants should utilize all forward-facing belted positions before using side or rear-facing positions.
ADULT BELT LAW
Oregon law requires that all motor vehicle operators and passengers be properly secured with a safety belt or safety harness, unless all safety-belt equipped seating positions are occupied by other persons. This applies to passenger cars, pick up trucks, motorhomes, and fee-based people transport carrying fifteen or fewer persons. Limited exemptions are allowed under ORS 811.215. Vehicle owners are required to maintain belt systems in working order.
Finally - here is a good presentation from California's Department of Public Health about RVs and recommendations for those with children riding along.