Thanks all so far.
The rental companies have been surprisingly unhelpful so far- still working on it but so far they either can't do it or don't understand the problem. I take your point re the front passenger seat but I'm pretty sure it's illegal for under 8s in CA. The SUV/trailer idea had occurred to me too but I have no experience with it- would that be logistically much harder to rent and to drive/park? Ignoring the cost (I assume its more).
Are there any class Cs with three forward facing seats in the back? Obviously 2 at the dinette- do any have another? Our 27 footer had a captain's chair but I don't think it swivelled to fwd facing and definitely no tether- not even sure it had a seat belt- probably not that safe..