Just to make sure, you do know that three-point belts are not required, don't you? Motor vehicles, including motor homes, are only required to meet the Federal Motor Vehicle Safety Standards (FMVSS) that were in effect at the time the vehicle was built. So if it originally didn't have shoulder harnesses, it doesn't legally need them now, either.
I don't fault you for wanting to do itó safer is always better. I just wanted to point out that you don't have to do it.
Engineering: Finding complex solutions to simple problems you didn't even know you had.