We stayed in Plitvicka (on a small site from the Camperstop Europe book, nearest one to the waterfalls - NOT recommended, it's a rip off) and on a site in Lovran. All I can say is that the road from Opatija down to Lovran was very narrow and twisty. I guess you'll be going down the A8/E751 to avoid the very tiring coastal route (66). Apart from that we have no experience of the Krnica area. We were there [Croatia] about 6 weeks ago
Happy & safe travels
Merv & Sandra