Be aware that your size will greatly restrict the availability of campsites in any of these parks, as they were built before big rigs were around. You night look at finding a place near the CG's to park your toad where you can walk back to your campsite.
Also, you might look at
Campsite Photos for the CG's you are interested in. You can see where others have parked big rigs in the photographs.
Start with Yellowstone as they allow you to book furthest in advance. We like Madison best for its central location. We are heading to Grant CG at YNP next week - we didn't reserve early enough! For your size unit, you might have to call Xanterra directly and speak to someone about the size of your rig and toad and see what they can do for you. Otherwise, you'll be outside the park in West Yellowstone. Not ideal because of the traffic backups at the entrance in the a.m.
Next book Glacier. You can reserve six months out there, so if you're looking at July, you can reserve starting in January. Again, rig size will be an issue. There are first come, first served sites at Many Glacier, but I've heard of people getting in line before 6 a.m. and still not getting a site! Apgar on the west side is entirely FCFS. If you want to see how early to arrive at a FCFS CG, check out the NPS
Campground Status Page. Click on a campground and it will take you to a page where you can see the historical fill times on a day-by-day basis. Very handy.
In Tetons, there are a lot of FCFS spots. Unless you're there at the busiest times, I've found one can usually find a FCFS spot.
