I found the coastal route between San Fransisco and Los Angeles to be stunning. A road right on the cliff edge by the water. Amazing views. I don't recall name of the route but it shouldn't be too hard to find.
In Europe I would suggest something like traveling in Cornwall in The UK or along the French riviera. Both are well worth a visit. Better in the summer or late spring though.
Pacific Coast Highway, Route 1.
It's gorgeous (and continues to be so well-north of San Francisco), but...
Don't do it in an RV! Especially a rented one.
It's windy and twisty and foggy and narrow. RVs are poorly aerodynamic, and most people are bad drivers of them. RV renters, particularly so.
A bit closer to home could be along the Norwegian coast. Some of the roads along the fjords are supposed to be amazing as well.