I assume you map it out first, figure how many miles a day you want to drive and then where to stay each night...right?
Not really. That's way too organized for us!
If we have a firm destination in mind (for example to attend a reunion) and need to be there by a specific date Jerry has an old computer version of Rand McNally that he uses to guesstimate the distance so we'll have an idea of the time required to get there, although we've now driven across the country so many times we have a pretty good idea about that. Then we consider the weather to factor in extra days in case we have to hunker down somewhere. We also think about any sightseeing along the way, but we don't usually plan an itinerary. I collect travel articles of places we might want to explore and have them organized by state so we always have places to visit. Before a trip we kind of look at a map and think what states we might cross so I can pull out appropriate bundles of info. But it's very casual. For example, if we're pretty sure we won't be in New England I leave those bundles behind, but I always try to have a map for each state in a wide swath because of our propensity to change directions.
Normally, however, we decide the general direction we want to go and start that way. We don't plan too far in advance because that removes flexibility. We sometimes change direction completely or partially because we see something that interests us. (We might head to Wyoming, encounter snow, and end up in El Paso instead.) And we've been known to stop at the campground exit and flip the proverbial coin to decide "right or left?"
We don't plan campgrounds either until about an hour or so when we start thinking about where we might stop. For campgrounds we tend to use the Trailer Life Campground & RV Resort Directory
. Another good thing if you're on an interstate is to stop at the state visitor center as you enter a new state. They often have great maps, booklets of attractions, and campground brochures. By the way, no campground resource covers every single campground - they pay to be included and when advertising budgets shrink, so do the entries in the directories. If you're really stuck Google "campgrounds" and the name of the town you want to stop. We've always been able to find some place to park for the night, so it's not a problem unless you're in cold areas where campgrounds have seasons and they shut down for the winter. Just avoid private property and obey "no overnight parking" signs.
Because you're new at this you may be worrying too much about all these things. Remember, it's supposed to be fun so don't obsess about it. Some people like military precision and others don't so you have to look at yourself and see what mold you fit and act accordingly. Also remember that if you lock yourself into a firm itinerary with lots of reservations and then something happens and you need to change plans, then there are lots of reservations to cancel. It's easier not to deal with all that.
I spent weeks planning our trip to Utah but that was our first major trip and I didn't want surprises.
By contrast, with our very first motorhome back in 1972 we got it one week and left the following week for a "summer trip around the country." All we had was a box of paper maps for every state. We didn't know where we were going - only when we needed to return. We changed directions a couple of times because various family members were moving and we wanted to be at their old place before they left or at their new place when they arrived. That trip was about 11,000 miles and we had a wonderful time with no planning to speak of. We literally circumnavigated the USA and made a few excursions up into Canada - tip of Maine to tip of Florida on the east coast alone.