There are two crossings you could mean. One goes via Čepikuće and the other goes via Zavala. Don't believe Google Maps, the route via Čepikuće is the major crossing, not the one via Zavala. Take a look at OpenStreetMap, which does a much better job in this part of the world - https://www.openstreetmap.org/#map=12/42.8500/17.8378 .
The Čepikuće crossing has been recently been upgraded but is getting more popular, and at weekends in August can have queues. It is a full international crossing.
The crossing via Zavala is a local crossing, but during the tourist season in 2017 and 2018, it has been open to non-locals. I have crossed in a Croatian hire car and in my own UK registered car without problem. Although this route appears the most direct it is not necessarily quicker to get to Ravno, since for much of the way it is single track with passing points where you may need to back up if you encounter someone coming the other way. It would be worth checking in Slano tourist office that this crossing is open in 2019 if you plan to use it.