Driving Munich to Dubrovnik in July

If you plan to stick to the main autobahn-style roads, Croatia is very good. There are not many roads, and the ones in Croatia are modern, well-designed, and easy to drive. There are plenty of gas stations, with associated restaurants-shops, along the highway. In 2011, I bought a bottle of pear rakeja at one - great stuff. I've not driven S of Split. Remember that in most European countries you need to buy the vignette for each country - the medallion which allows driving. Don't skip this.

That's a very long drive, so you might consider doing it over 2 days. Plitvice is just over half way and, while a slight detour from the motorways, would be a good overnight stop. You could visit the park early before the crowds arrive and then continue on to Dubrovnik. I had thought about suggesting an overnight in Ljubljana, but that still leaves a long drive the second day. I too have not driven south of Split and don't recall whether the planned southern motorway has opened, but I believe at some point you'll be relegated to good 2 lanes roads. Even the rural roads I've driven in that part of Europe are well maintained, so roads conditions should be good the whole way.

On the subject of vignettes. Yes, you need one for Austria and Slovenia, but not Croatia, where you pay a toll to use the motorway instead. In Austria, in addition to the vignette, you also pay a toll to use the longer tunnels.