Best guide I've found, and I've bought several, is "EZ66 Guide for Travelers" by Jerry McClanahan, published by the National Historic Route 66 Federation. It covers various vintages of Route 66 (routing changed over the 30+ years it was so designated) and helps you find historic relics long abandoned.
"Historic Route 66" markers through Oklahoma mostly put you on what is left of the last routing before US-66 was decommissioned. There is a lot of useful mileage of this vintage, as I-44 (routed onto the Will Rogers and Turner turnpikes) did not replace US-66, rather bypassed the old roads.
You might be tempted to get onto expressways to go through Tulsa or Oklahoma City (you will have to to get across the Arkansas River) but following the later routings through town can be worthwhile, although 66 did not go through OKC, it goes through Edmund, to the north. In Tulsa, the historical markers take you across 11th Street, where you will find Talley's Diner at 11th and Yale, a 50's style venue that was not really there in the 50's. However, I'm not sure where you would park a motorhome, it is hard enough finding parking for a minivan in that area, particularly when the classic car guys or bikers are gathering at Talleys.
I used to prefer the Metro Diner on 11th, but the University of Tulsa bought the property and tore it down to build dormitories. Presence of a growing university community makes this a high-rent district, and is also why parking is hard to find.
I used to have a favorite lunch stop in Stroud, but I think the property has been turned into an upscale bar, kind of like an unfranchised Hard Rock Cafe.
With a really good guide, stopping at all the attractions, seeking out the old bridges and byways, it could take you a week to get from Joplin to Oklahoma City, nominally 3 1/2 hours on the turnpikes. Turner Turnpike was built to cut the Tulsa-OKC travel time by half, and with the Will Rogers it pays for the whole state turnpike system, as all the other segments don't have the traffic to pay for upkeep.