I would suggest a rear facing seat would be damned uncomfortable for a three year old on a journey. The height of a three year old would see him/her with legs bent and squashed. Trust me, I know, we have a three year old grandson. Seat belt attached car seats are fine, but you must try before you buy, they dont suit all car seats. Example. In our Shogun, we have two identical seats, one with harness and one with harness removed (as in high backed booster) utilizing the adult seat belt. With the Shoguns seat in normal position we find the seats, even with belt lock are sloppy, but if we recline the rear seat one notch, fit the seats and then push the rear seat back up a notch, the seats feel like they have been cemented in place. Many seats nowadays, even Isofix seats either have a bottom bar that rests on the cars floor pan, or a top tether, no use at all in a motorhome. Of the three types, I would say the seat belt type are most suitable for your needs. Also, a child over the third birthday is coming to the natural life end of a rear facing seat.
To end, in the spirit of levity started by Mel, why not put the kids in one of those Zorb ball things and just let em roll around.