The fairgrounds does have camping areas. There is a North Woods area plus upper and lower campgrounds, and a recently completed Central Buildings Bath House added showers and restrooms. What the fairgrounds publishes about using them is deliberately conditional, and we are going to quote it rather than improve on it: camping is accommodated during certain events, sites are not always guaranteed and for most events are first come first served, and water and sewer are not guaranteed with camping.
No nightly rate is published. No electrical amperage is published. There is no online booking form. Whether camping runs during the fair itself specifically is not stated anywhere on the official site, because the policy is set event by event. The only way to find out is to call the fairgrounds at 417-833-2660 and ask about your dates. Do that before you book anything else, because the answer changes the whole trip.
If the answer is no, the 25-mile drive from Marshfield stops being a detail and becomes the plan. Plenty of Springfield area parks take a trailer, and a half-hour run each way is a normal commute for a fair that shuts its gates to new entry at 9:30pm anyway. Tell us which park you end up in and we deliver there instead. Either way you are not towing anything.
"Rented a Class C for a family trip to Silver Dollar City. The RV climbed the Ozark hills with no problem. The kids loved the bunk beds, and the pickup in Springfield was super quick."