Man of Honour
Two cars to fuel is probably not going to be much, if any, more expensive than fuelling one RV. My experience of RVs is limited but my parents have owned a campervan and borrowed another (much larger one) and in both cases the fuel economy was pretty shocking but then again what do you expect when you're driving something with all the aerodynamic ability of a garden shed at 55mph or whatever.
You might want to look into the hire costs as well but it's perhaps not all that clearcut in terms of pricing, you'll save some money on the costs of hotels/motels with the RV but probably will find it to be somewhat more cramped and in some ways offer less freedom.
I've done some of the Pacific Coast Highway and while it's certainly possible to do it in an RV (or at least much of it) it would seem to be considerably less fun than doing it in a convertible. It's also worth bearing in mind the roads can be very steep, twisty and in quite poor condition.
![[TW]Fox](/data/man-of-honor/2009.png){kind=avatar}
. Some British charm and negotiating got that to $9 a day extra, well worth it! Unless i was just unlucky on my timing and hertz personnel then the easy way if getting a Stang seems to have been made less so these days!