I've done a lot of research on this topic because we considered getting a motorhome and we have two young children still in carseats. Honestly, there is no safe place in an RV to put a carseat. Side facing is definitely not safe because car seats at not designed to protect when side facing.
And in majority of motorhomes (actually, I havent been able to find any that are not like this) the only two seats that are actually connected to the steel chasis of the vehicle are the drivers seat and the front passenger seat. All of the other seats are just connected to the floor.
I was really bummed when I found all of this out, as I was seriously contemplating getting a Class A....or possibly a C. This has deterred me though. I'm a real stickler for car safety as there are many crazies out there on the road. I dont want to ride in a vehicle that is not safe for my children. I do understand that the larger the vehicle, the safer you are.....but still, it does not set my mind at ease.
The "safest" option would be if you have a forward facing captains chair that can be locked into position. Though, this is still not a very good option because you cannot usually use the tether on your childs carseat....and also, the chair is probably not connected to the RV's steel frame. Plus, the cushioning on the chair is a problem as well.
:::sigh::: I wish they would make a more "safe" motorhome for families. For couples it is fine as they can just sit in the front drivers and passengers chair while traveling.