I repeat this because when checking both a couple of GPS as well as asking a few locals (or maybe not?) in Khao Kho, I was told to go to Highway 12. In other words, if I had not already visited this national park, I would not have found the correct entrance.
This visit was during the hot and dry season while my first visit was during the "cold" season:
http://tinamue.blogspot.com/2010/01/thung-salaeng-luang-national-park.html
All in all, the dry season is not an interesting time to visit. Next time, I will try to make it during the rainy season as the tracks leading through the park then (hopefully) will be muddy, challenging and fun to drive.