We have driven several times between Colorado from northern Illinois on I-80 and I-70. We find the traffic and drivers on the interstates make for a not so relaxing drive. The last time we went to Colorado, we decided to take US Route 36 from Springfield, IL to Denver and found the drive to be soooo much better. There is much less traffic and the road is in great shape almost the entire route. The speed limit is 65 on most of the route, which is great, because we don't like to drive faster than that while towing and can still make good time. It takes about 2.5 hours longer to get to Denver, but that's a great trade-off for a much more relaxed drive.
FWIW
FWIW