Google Maps shows Cincinnati to Fishing Bridge as 1660 miles. At 3.5 consecutive days, that's an average of about 470 miles per day on unfamiliar roads, which I would find a fairly exhausting stretch (even in our small Class C, not towing.) Do you have more than one driver?
As others noted, you can't really count on driving at the posted speed limit when estimating RV travel time. You also might want to factor in potential delays for road construction and lane closures due to accidents. If you add in stops for fuel, food, bathroom, you are looking at a run of 10-hour driving days even if you have all smooth sailing.
Can you add one more travel day in each direction? It would still be a marathon to get there, but closer to being feasible.
Fishing Bridge makes a good base inside the park. Have a great time!
