Croatia and Slovenia both superb, we've been for the past two years. The first time we came from Venice, through the tiny coastal bit of Slovenia and then down into Istria, returning through Slovenia into Northern Italy, into the Dolomites and back from there.
Last year we spent several weeks in Italy, driving down as far as Bari where we caught the overnight ferry to Dubrovnik, returning up the Adriatic coast, via a bit of island hopping and back essentially the same way through Slovenia.
You'll have no problems, both countries are extremely well organised, roads generally good, fuel dirt cheap, supermarkets and local shops plentiful, and best of all stupendous scenery.
I agree with Grizzly that the huge ex-Soviet campsites are soul-destroying, but they are easily identified by their sheer size and there are plenty of others.
Roger