I've taken one extended RV trip. The first step in planning was to call up my dear mother, who also loves to travel, and saying "would you be interested in joining me on an RV trip to Alaska?"
Whether or not you need reservations depends mainly on where you are and when you're there. Some places and some times it's simply not practical to rely on space availability without a reservation: some of the popular national parks, holiday weekends, Florida during prime season, etc. Much of the rest of the time and country, it's entirely practical to wing it, or at most make reservations only a day or so in advance (or on the same day).
Some people are not comfortable unless they know where they'll be headed, and reservations are good for them. Some other people are not comfortable unless they know they have flexibility to be spontaneous, and reservations are more of a hardship than a help for them. Similarly, how far you go in a day is rather up to your personal tastes; 300 miles in a day is entirely reasonable for me, but not for everyone. It also is somewhat dictated by where you're generally going on the trip, and over what timeframe, and of course vice-versa.
I would start with a very rough idea of your itinerary, and figure out what popular places you don't want to miss (ones where reservations are necessary), and about where you'd be for holiday weekends and the like (ditto). Planning out approximate stops per night can be helpful, but I personally would avoid chiseling that in stone too early on. It is good to leave some flexibility if only for the possibility of unexpected problems (you need vehicle repairs, or a key road on the route gets flooded out, or high winds and thunderstorms prevent safe travel).
