Best route Kansas City to Harlingen, Tx with 5th wheel?

I am sure Padredw will verify it later. But I do know that US 281 is two lane from Wichita Falls to Lampasas, and then 4 lane undivided to San Antonio. Don't know about south from there.

I will add that 281 is a good highway, mostly rural, very little traffic, and certainly more scenic than an interstate. I often travel the section between Lampasas and Johnson City and frequently see RVs of all sizes going both north and south.