by 3 days if you mean being in Pittsburgh at the end of the 3rd day this would work:
1st stop - I-40 to North Little Rock KOA - 7 hours.
2nd stop - from Nashville, north on I-65 to Kentucky exit 53. Cave Country RV Park. (call ahead to make sure they have a site). 6.5 hours
3rd stop - from Louisville, I-71 north to Cincinnati, then I-70 into Pittsburgh. 7 hours.
If you mean 3 nights and the 4th day in Pittsburgh, then we can adjust the route and stops accordingly. I think the above route is very aggressive.
On the return trip, we usually take I-77 south to I-81 but there are steep grades and tunnels if they don't bother you, it's a gorgeous drive! I-81 to
I-40. In Dandridge TN is Anchor Down Resort. Or if you have more time, we like River Plantation in Sevierville TN.
From Sevierville, take I-59 to I-20. Before you get to Meridian MS, take US45 north about 4 miles to Benchmark Coach & RV Park in Marion MS. Then from Meridian MS, take I-59 south to I-12, Then I-10. If you need another stop, we like Lakeside RV park in Livingston LA off I-12. Call ahead for pull thru.