Generally speaking... the internet. If you do a Google search for "Class A RV and Car Seats," you'll find a lot of threads and websites pointing you away from using a Car Seat in an RV. Most say to have your child in separate vehicle and not in your Class A or C motorhome.
Some people have placed car seats in the dinette, but it's frowned upon unless the seat belts are bolted to the frame. Others have put them in side-facing couch seats. But, this is even more dangerous! Others have suggested placing the car seat in the front passenger seat because it offers the most secure seat belt. But, there again, you're not supposed to place a child in a front seat of any vehicle.
This is becoming a struggle and a bit disheartening. We were really looking forward to traveling with our son in a Class A, but with all the signs pointing away from a Car Seat in a motorhome, we aren't left with many other options.
https://thecarseatlady.com/rvtravel/