Well darn. I had a long post for you and the website dumped me before I could post.
Well ... okay ... this one is long too.
In addition to what Loneoak said ... I've used GoodSam Camping, ParkAdvisor, and KOA apps to find campgrounds. You probably don't need KOA, unless you're a member, as the other apps will include the KOA sites.
I'm cheep and hate paying the inflated prices at the truck stops. I use the GasBuddy App to find diesel. During our travels we'll start looking on the app a calculated distance ahead when the tank approaches 1/2. Most stations will have awnings that are higher than your rig, so height typically isn't an issue. The issue becomes your comfort level with navigating tight locations with your rig. We've had some, "We probably shouldn't have stopped here." moments, but we've never damaged our rig or anyone else. ;-) Truck stops are definitely the safest, but come at a cost of higher prices.
We typically plan our trips with 400 mile legs. That typically turns into 7-8 hours when we're towing given a couple extra fuel stops and slower traveling speeds. We love to drive, but don't relish the thought of setting up in the dark in spots we haven't been before.
If you are planning to stay at private/commercial campgrounds and not BLM, national park, or boondocking, you might consider week increments as that will be more economical. e.g. It'll be easy to eat up a week in the Yellowstone area with Jackson Hole, Grand Tetons, etc. And you might want a day or two to relax without driving too. Obviously you can't do that all the time, but consider it if you're planning on more than a few days in any one location.
Longer reservations are tougher to get in popular locations. We didn't have any problems finding places for one night, but several days, a week, or a month get difficult in some locations. Plan several months in advance if you can.
You might find some valuable comments on this thread ...
http://www.montanaowners.com/forums/...ad.php?t=77279
Be safe and have fun.