259 in Eastern Oklahoma from Broken Bow to Heavener, 59 in western Arkansas from US-70 north to Heavener, you will be crossing the Ouachita Mountains, against the grain of the land. There is no highway across these mountains built to Interstate standards, it will be hilly and curvy, and in some places slow going.
Of the two routes, 259 is the easier because the mountain core is more worn down to the west. US-59, particularly the scenic drive out of Mena, has some switchbacks (not tight, sized to the largest legal semi-trailer trucks) and some steeper grades (short sections maybe exceeding 12%). It is a lovely drive in a sports car, and a great motorcycle road, but it is one I avoid in the RV because I'm just going to be holding up the people driving it for fun.
If you want high-speed roads (75 mph limit), sweeping curves, no grades over 6%, work your way up to Paris on those good East Texas two-lanes, take US-271 up to the Indian Nation Turnpike (which skims the edge of the Ouachitas), get off at US-40 for Fort Smith. That is all at worst what gets called "rolling" countryside.
Tom Test
Itasca Spirit 29B