Thanks OP for the update. I do agree...very little info is out there on seat belt usage or non-usage in motorhomes because the insurance industry has not tested or been asked to test. It could be at some point in time the gov will require DOT standards for all passengers. But for the time being are out there winging it on our own. In our rig we have stickers all over the place, as told by the dealer--CYA "This seat not for use while in motion"!
I would suggest if anyone has worries/issues they should contact the manufacturer to see what ideas they can offer.
My biggest concern with MH seat belts is how they are attached and where. I've taken off the cushion and the base top of the dinette to pull up the belts and clean under there. I found out the 2 belts on each side were mounted to ONE bolt therefore the belts straps were just made longer to reach both seating locations. I could not say the bolts are the proper quality but with the belt straps so long I'd think there could be an issue in a sudden severe movement where sliding back and forth would render the seat belts so loose they'd be very ineffective/virtually useless. However I do believe the situation could be remedied...3 additional bolts put in on each side with proper mounting. I can't speak to whether all manufacturers so this or just ours.
The couch is the same...we have 2 sets of belts...each set is mounted to one bolt. With a couch on a slide out it may be more difficult to mount each on 2.