I usually go up SH 37 to near Idabel then take the 70 bypass north at the 4 way stop sign. This will take you around to Hwy 70 north to Broken Bow. Not sure where it would be best for you to hit 37. Don't know if your taking I-30 east or going up through Paris. If I-30, you will hit SH 37 at Mt Vernon. BTW, no need to stop at the office when you enter the park. Probably no room to park anyway if you have a large rig. You can continue right on around to the camp loop. The reservable will be the first loop you come to. First come first served is beyond the reservable section past the tennis courts. Good luck and have fun!
Also, when you pass the park office, there will be a 3-way stop adjacent to the office. Continue on straight to get to your area of the park. If you turn left, you will miss the RV loops. Come to think of it, there may be a few RV camp spots in the other section of park, but really not sure. Its been a while. Sorry.