Is what's "legal" always the safest- or even the most important- factor?
As a Grandma, I'm much less concerned with the "rules/laws" related to child transport than I am with the safety of the kids themselves.
The "rules/laws" are based on extensive testing and research. Unless you know more than they do, I'd take their advice. The same laws of physics will apply in the RV as in a regular vehicle. I don't think second-guessing what the research has proven is a good idea, i.e, kids should be in a forward-facing seat. I've no doubt the OP has the best interest of her children in mind, otherwise she would not have posted.