You'll find that people take various approaches.
I usually start with Google Maps to pick a route. DW prefers no more than 6 hours in the truck, so I'll look for parks on the route that fit that requirement. If towing mostly on interstate highways, I'll assume an average of 50-55 miles covered per hour (that includes short breaks every 2 hours, one of which will be for refueling). If secondary highways, my assumption is 45-50 miles. So depending on the highway plan, 6 hours is somewhere between 270 and 330 miles per day. Keep in mind that most trailer tires are rated for 65 mph MAX speed and exceeding that can lead to blowouts. Also, at higher speeds, bumper hitch trailers can become unstable and experience excessive sway. So don't plan on going faster than safe towing allows.
Figuring the distance leads to finding cities/towns on the Google map. I then jump over to
https://www.campgroundreviews.com/
to find the parks in and around that location. I try to stick with parks fairly close to the highway. Amenities are listed and the listing may have photos. The reviews are from other campers and will give you a pretty good understanding of what you can expect at a given park. I read both good and bad reviews. There's usually a link to the campground website too.
There's some iteration as you plan the trip. After finding the 1st stop, when looking for the 2nd, the distance might be a problem and the 1st selection gets changed.
When you have your stops planned, start contacting the parks to make reservations. Most want a credit card to hold the reservation. Some want payment at the time of reservation. Be sure to understand the change/cancellation policy for each park - they can vary quite a bit.
Anyway, that's what I do. Others take a different approach and use different information sources. Some people look at nearby attractions and spend a couple of days at each stop. Some don't make reservations, but occupancy is higher than ever, so this is getting riskier. Some people stay at Walmarts - but check if the particular Walmart allows RVs overnight - some communities prohibit that practice.
There are more ways than what I've described. I'm sure others will share what they do.