Didn't feel like it at the time since I discovered my car had a cracked engine block and had to buy another one. But yeah, given gas prices, that would have set me back quite a bit more this year.
I'd consider just driving and getting AirBnBs along the way when you do this. We did that instead and it gave us more mobility when we got to where we were going and some of the places we stayed in were really cool. I think it was probably cheaper, too, although I had this dream of driving the RV.
Also, if you want to do a mini-version, you could fly into Rapid City, SD and rent a car and in 4 days hit Mt. Rushmore, Custer State Park (not to be missed), Deadwood, the Badlands, and Devil's Tower. That's a really cool area.