If you don't have it already, purchase a GPS. These will suggest routes and distances. Once I've a route roughed out, I go to Good Sam Club
to see Good Sam's rating of the parks in the area. I then check candidate park ratings at Reviews of RV Parks and Campgrounds - RV Park Reviews
. What I often find is that the distance I'd planned to travel is altered once I start looking for RV parks. We always reserve a site. How far in advance is determined by the time of year and popularity of the area.