Do not use the couch. The sideways whiplash would be horrible for a small child.
DO NOT face a child toward the rear unless ALL missile hazards are removed. Everything must be stowed. Stove top covers, dinette chairs, lamps, everything on counters, you get the idea. In a sudden stop, those items that are not firmly secured will be travelling at crash speed towards your child's body. Think about a corian stove cover or toaster oven or coffee pot travelling at 60MPH toward your grandchild.
I use the front passenger seat. I put in extra seats belts anchored to the frame below to secure the child seat and I also have a rear tether to keep the child seat from flipping forward.
RV couches are not usually bolted to a frame to secure them. My sofa is held down with about 4 long wood screws and the seat belts below them are just as bad. We don't use them for travel as they provide no safety.