Upvote:8
The ADAC provides a nice map that allows for route calculation, but also displays all the campsites along a route. I checked your route from Antwerps to the Tatra national park and basically, the campsites along the route are so dense that it doesn't really matter:
In Poland, there is basically every 50 to 100 kilometres a camping site directly next to the high way. In Slovakia and the Czech Republic, the density is quite similar, while in Germany, the density seems to be even higher. So I wouldn't worry too much and choose the route based on other criteria such as sights along the way or huge highway construction sites which you can also check on the ADAC map.