I regularly take I-35 out of KC, across US-36 to Hannibal, I-72 to Springfield, I-57 to US-24. Once on 24, it depends on my destination.
For Western Michigan I'd usually go up toward Michigan City, using either I-94 or the Indiana Toll Road, depending on what I've learned about construction projects, to an artery going to my destination (whether Kalamazoo, Battle Creek, Lansing, etc). For Elkhart area, I'll take US-24 to US-31, then across US-20 to Elkhart, Shipshewana, Middlebury or LaGrange, or across US-6 to Nappanee or Wakarusa; US-6 gets you closest to Goshen, I suggest SR-19 north out of Nappanee then SR-119 into Goshen.
SR-25 north from Logansport to US-31 at Rochester shaves off a few miles, but I don't usually take the RV through the center of Logansport, and even with just the car getting bogged down in traffic can cost more time than it takes to get on the US-24 bypass and drive the extra miles.
The only real bottlenecks are Monticello (during tourist season and on market days) and Logansport (should you not bypass it). Watseka has built up considerably in the past 15 years, but I am usually through it in about 12 minutes, but wrong time on the wrong day can make that as much as 20 minutes.