My method is to use a route planner (your choice: Google, BaseCamp, various other PC or pad/phone based apps).
1) Give it your starting point and ultimate destination. Change the routing as desired to go through or avoid places along the overall route.
2) Then go to the detailed turn-by-turn directions.
3) Choose your preferred miles per day (for instance, 300 miles)
4) Look at RV parks around the steps in the detailed directions. These directions usually give the mileage for each leg in addition to the cumulative mileage from you origination point. Therefore, in this case look at around 300, 600, 900 etc. cumulative miles. Use an RV park search app such as RV Parky, RV Park Reviews, ParkAdvisor, etc. around the 300-mile mileage intervals.