I've seen this discussed in several venues and unfortunately there seem to be no good answers.
On the legal side, state and provincial laws vary in their requirements. Be careful when citing laws because large vehicles are exempt from the passenger car requirements in many jurisdictions. For example, buses [even school buses!] usually don't even have seat belts. The exemptions may be motivated by cost issues, but it is also true that passengers in large, heavy vehicles are at much less risk than smaller ones.
But the safety of your own child is a strong motivator. Clearly sideways facing seats are out - in an accident they are dangerous for anybody! A front seat position on a motorhome may not be a bad place - the windshield is far away and the seat is above the "crash zone". At least you could solidly strap in a car seat there. But that won't pass legal muster in states/provinces with "no front carseat" laws.
Most dinette seats are not strongly constructed, though some are equipped with seat belts to meet RVIA and legal requirements. If you have a dinette, that's an option.
I don't have an small children or grandchildren, but I believe my choice would be to install anchors in an open area, in the aisle if necessary, and place the car seat on the floor. It gives a solid base and good anchor points and the seat can face rearwards, which I believe is safest style. If the anchors had to be in a foot traffic area, I would find anchor points that fold down flush, like the fittings used on sailboats, and buy or make webbing that connects to the anchors with a snap of some sort. There are snaps that are plenty strong enough, e.g. parachute equipment, military or race webbing, etc.