Jerry,
For the most part kids do not sit and look out the window in these RVs from our experience with ours which may make the couch an option. They have other interests while driving, a lap type DVD player or a hand held game is more likely, possibly a book, but all the kids are different. Our last motorhome had seat belts bolted through the floor and the couch had slits in the bottom near where the base and back join where they were worked up through and could be tucked in the crease below the back when not in use. You could possibly have the slits added. If the couch frame was a strong metal tube or rail and I used one of the couch anchor bolts for a seat belt mount I may use the couch frame for the other belt piece. If the frame is inadequate you may have to find drilling points that work for bolting and use a plate on the underside to spread the load.
If you go the captain chair route, they have swivel boltable bases. I would guess that you will have to use some appropriate steel plate or u channel on the underside of the slide floor to fasten those chairs and seat belts adequately. Beaver Coach Sales may be able to give you some insight, they must have added seat belts over the years.
Later Ed