I make the trip in my motorhome from the central coast of CA to Oshkosh; great trip but long. I take I15 north than I70E. (I don't like the climb around Salt Lake) Just before Denver, pick up I76E pretty easy. Then, into Nebraska and pick up I80E. Take I80 east to Des Moines and head north on I35 to pick up I90E. This keeps you out of Illinois; I80 can be a mess in Illinois (I'm from Illinois; been there, done that). Take 90E and you will have to take a short jog up to 21 than straight shot into Oshkosh. Lots of different routs; this one has worked for me many times. I have my stops along the way and never pay for any camping the entire trip. Great way to travel and helps off set the cost of gas. With no camping fees and eating all my meals in the motorhome en-rout it helps a lot with the cost.
Marty