Mr331, don't loose heart. Be a little spontaneous and travel without having everything set in stone.
Yes, your rig is Large, but Mammoth CG (north entrance) has many large sites which can accommodate Big Rigs.
We stated at Mammoth CG in 2009. Didn't need a large site, didn't take one. But there were many there, some had long TT, others had some good sized Diesel Pushers. Can't reserve any (First Come, First Served).
Mammoth CG <-Check the More Information clicky . "Vechicles up to 75' can be accommodated."
We have been to Yellowstone 4 times, never had a reservation. What I have on
Yellowstone & Tetons.