KRCM, just a little ways east/southeast of Kansas City has had burgers and hot dogs the past couple times I stopped there on my way to Oshkosh as a mid-trip refueling stop. K81 (Miami County) just south of the KC mode C ring has the We-B-Smokin BBQ restaurant on the field. They're closed on Mondays and only serve breakfast menu on weekends from 7am until 10:30 am but serve BBQ after that. Their omelettes for breakfast are awesome. I used to stop there each year for breakfast on the way to Osh, and for BBQ lunch on the way home back when I still had the Cherokee. A bit closer to Osh, is Portage WI (C47) with good fuel prices, mogas, and brats served if you want to fuel up and get a bite to eat before hitting the Fisk arrival. One thing cool about Portage is that a GPS direct course from C47 to KOSH takes you right along the south shore of Green Lake and into Ripon to set you up to start following the railroad tracks to Fisk.
ETA: found a website that said KRCM will have burgers, hotdogs, and BBQ sandwiches from 11am to 1pm each day during Oshkosh week plus a 10¢ per gallon fuel discount if you mention you're an Air venture traveler.