The toll from Eastern entrance to the Topeka I-70 exit for a RV towing a car is $ 6.00 cash or $ 4.45 if you get a free K-tag sticker for your RV and set up an account. ksturnpike.com
. It's worth it comparted to the non-toll routes.
I suggest I-35 to I-29 north to I-635 south to I-70 west to K-156 southwest ...
Alternate with no toll would be to go thru St. Joseph MO and see the sites there then cut down to I-70 at Topeka.
Some possible stops include Topeka KS (State Capitol, Kansas Museum of History), Abilene KS (Eisenhower Library, Historical Museums, really good food at Brookville Hotel), Larnerd KS ( Ft. Larnerd National Historical Site )