Great Question - I am sure you are going to get many great answers. What resources you need will will depend on your travel comfort level, that is, do you like the comfort of knowing you have reservations for all or most of your trip or are you a spur of the moment type of traveler and if you can't find a open space no worries you will move on. Here is what I find I use the most on traveling -
1) The Good Sam online trip planner - trips.goodsamclub.com takes the place of phone book size book they used to publish with all the campgrounds.
2) The latest edition of Next Exit I get one every year and we use it every day for fuel stops, lunch stops, etc.
3) FMCA (Family Motor Coach Association) road Atlas - You always need a good map - shows Walmarts, Flying J's etc. I use it to follow our travels every day .
4) Flying J App - has a trip planning feature and if I am not on the interstates highways I use the Gasbuddy App to search out reasonably priced fuel.
5) A Good Sam or other brand GPS made for RV's so you can program your height, weight into and avoid roads or situations where you do not fit. A good GPS also will show you what is at the next exit on interstates and will show campgrounds when you use it trip plan.
6) regularly I fall back to the Google Maps website maps.google.com I use it preview my route, and at times I have checked Street View to review some hairy intersections and turns as well as what to look for such as campground entrances, etc. The best feature is I center the maps over an area where I plan to be at nightfall and then do a search for "campgrounds and RV parks" and they all show up as red dots where you can click on to get more information. I also use when I get to the campground to locate nearby Grocery Stores, Churches, etc.
7) Tripadvisor App - I use it to check for restaurants, campgrounds, etc.
8) the website Restaurants on TV, Diners Drive-Ins and Dives, Man vs Food, Best Thing I Ever Ate & more
to look for those Diners, Drive-ins and Dives that I don't want to miss
9) the website Roadside America - Guide to Uniquely Odd Tourist Attractions
to look for those odd ball things in an area that I am at.
Most of my trips have been made during the summer months and I made reservations for most of the nights. With that said I found outside of a few National Parks or high tourist areas the places we stopped at had many openings so reservations would not have been necessary. A friend of mine does his thing in the winter traveling for two months during the Jan/Feb and they stay a week here and a week there. The high touristy areas - National/State Parks, Lake Havasu , Las Vegas, and the Texas Coast he makes reservations well in advance the others he will make them on the road usually the same day or day in advance.
Bottom line plan on using the Internet on a regular basis to plan and update your trip. Perhaps look into a Mifi or a Wifi external antenna / Wifi booster so you have always an opportunity to access the Internet.