On the road....coast to coast (4 times) my experience(s):
Leave early like 6-7 AM. Pull in to a chosen cg around 3PM. No problems getting a site for the nite.
Never even try for a spot without reservations if a holiday weekend. Especially within 100 miles of a major city.
Watch the weather. Major snow/ice on, say I-40...the RVs tend to move south to I-20...then to I-10. One would need reservations then. Figure out where you might be at the end of your day...and lookup campgrounds (Trailer Life Guide, for instance) and call. You should know this info when you leave for the day. Call ahead.
On our trips E-W or even N-S, we seem to use KOAs quite a lot. Then seem to have the interstates locked up...easy in, easy out and for the most part not bad places....PLUS most of the sites happen to be pull-thrus. (No guarantees, tho).
Our "comfort zone" is 300 miles a day. 6 hours. Hope this helps a little bit.