Lots of resources - some free, some not, some free sorta. I use a combination of Good Sam, Allstays, RV Park Reviews and Trip Wizard. Allstays is the most comprehensive and most configurable - but the interface is clunky. RV Trip Wizard is fee based but very comprehensive for total trip planning. Good Sam interfaces with my GPS. And finally, RV Park Reviews has the best (IMO) insights to each RV site.
I use the multiple sites to give me the best comprehensive plan. Then again I am a bit annal retentive about routes. (Don't like sheer cliffs, switchbacks, tolls, etc).
Hope that helps.
US Army Retired - loving it in our Challenger 37TB.