We had two runs at Route 66 one from the West and the other from Chicago.
We have hired RVs with www.cruiseamerica.com and www.elmonte.com both were ok with no problems on 1 month long periods.
The last one we did was through www.cruiseamerica.co.uk (The Motorhome Experts based in U.K.) who in the late winter early spring offer re location special rate one way hire of new RVs from Chicago or Toronto to destinations across the US and Canada. These are special reduced rate hires to take brand new hire RVs produced in Indiana to the hire depots.
The last one we did was in 2012 from Chicago to Las Vegas , we had a month to complete the trip and covered a fair bit of Route 66 including off route to include Memphis and Graceland as well as other off route diversions.
Incredible experience not to be missed.
I've lived and motorcycled all over the USA, wife pillion, on a Harley. I have been in major cities and deserted Badlands, including the 66, loving every minute. Out of all the rides, the mid-west has always been our favourites, fantastic landscapes and views. From Nevada to Arizona, Colorado through Utah and Wyoming, magnificent. What I would say, is if your visit is to be the first and possibly your only one, then Route 66 is not a great choice. It generates nostalgia and a cult following which can disappoint. You will I'm sure know that much of the original route has been replaced with Freeways, leaving sections of the old road intact. There are still remnants of old dwellings and towns along the route which are interesting, but also some long stretches of boring road. The complete route takes you on and off the original road, laced with stretches of newer major roads. I would advise that you take a look at all of the major points of interest and perhaps tailor a trip to encompass your favourites, there is so much to see and experience that the 66 wont come near. I have just returned from a trip beginning in Denver and including Yellowstone, a NP that was on the list to visit. A holiday in a camper or motorhome would be terrific, one you will remember forever, but don't be limited by the 66.
If you want to see the route, including the old bits that have been by-passed, just look up The You Tube Van lifers "Tread The Globe", they have travelled down from Canada to Mexico.
They visit all the museums & touristy bits and give a good idea of what to expect, warts & all. LES