I think your time estimates are optimistic. For trip planning we use 50mph overall average which equates to 10 hours for 500 miles. If I know we won't need to make any stops, I can cut an hour off that, but not much more.
I would take 79 across Indiana (skipping the 80/90 toll, which will be about $70 just for Indiana for your RV), then head through Peoria, IL to meet up with 80 in Iowa.
80 through Nebraska can be a bit dull, much like Kansas, but has more populated cities along the route. I think it would be a good alternative to 70.
2017 Winnebago Forza 38W