Gardinar, MT to Cody,WY

should not be a problem. The road gets a little twisty near the dam but nothing extreme.

212 is fine, but be aware that 296 IS the Chief Joseph highway :)
I think your two rigs can do it ok. There is a switchback climb to the top of Dead Indian Pass, but the road is not bad..you can get around those turns ok.
Just take your time and enjoy the views.

X2 on what TexasShadow said. We did the same roads in our Class C last June, with the exception that we turned north on 120 to go to Billings. I do recall using quite a bit of second gear going up the switchbacks to Dead Indian Pass and may have used second some going down the other side.

It's a great scenic drive! Stop the parking area at the bridge a few miles before the climb up to the pass and walk out on the bridge for great views up and down the canyon. Can't recall the name of the river below.

There is a small museum at the dam too and a good overlook.

Yea, did know that 296 was Chief Joseph Hwy. Haven't checked out enough yet, but didn't know about the dam or the museum, good info. Plan to stop at most overlooks (RV or car), especially cause I can't (or suppose to) look while driving. Thanks for info.

I don't recall any dam anywhere along the route the OP posted. There is a dam with small museum at the Buffalo Bill reservoir a few miles west of Cody.