We're 'old school'. What I used to do many times was to take a paper road atlas. On each state there would be a scale ruler at the top or bottom of the page... something like 1" = 200 miles. I'd take 2 fingers and guesstamate 1" and move my 1" finger measurement (or a ruler) along the highway for the amount of miles from my last planned stop to how far I wanted to drive to the next stop. That's where I'd start looking for a RV park. It didn't have to be exact. Look on a campground review site under a couple of the towns in the area and get a couple options. It's not perfect but it worked.
That said, if you only want to drive a specific amount of miles per day it's hard to stick on schedule. You could have vehicle issues. You might want to stop for siteseeing that looks interesting. You might get tired and want to stop earlier. You might have energy and want to drive more miles. Reservations really lock you in. You blow one and it throws everything off. We were fortunate that we rarely made them because we drove secondary highways where the masses of people weren't looking for a campsite. We also weren't particular as to where we stayed. It didn't have to have all the 'frills'. RVs are self-contained so you don't even need hookups, especially for a one-night stay. Boondocking was great for us. We always found something in the small towns - small city campgrounds that aren't even advertised, etc.
Since you're planning to drive short days, plan to pull into a RV park along the interstate before 3pm and you'll get a site without having to make reservations. Have a couple ideas in mind in the area.
Just an option for you if you want to be more flexible.
When you find a couple towns where you want to stop then check out this site for what's around there. It's a good site if you want to read reviews or find campgrounds:
https://www.campendium.com/
Now, if you want to stay at a particular popular scenic attraction or popular national parks you have to do a little more planning. However, just for traveling the interstate for 1 or 2-night stays, be flexible.
Not everyone travels with a GPS or mapping/travel computer program.
How do you think people traveled before all the technology?