Badlands to Custer in 40 foot rv, I-44 ok?

yes. it's two lane, but a good road.

No problem for your motorhome. You stated I-44... it's not an interstate. It's a good two-lane. Enjoy!

How do you plan on going after you get to Rapid City on 44, I certainly would not take 16A south from Keystone, not in a 40' & towing. there are too many of these.

Unless you just want to see a lot more rolling grassland, I'd go back to the interstate and up to Rapid City and then to Custer.

In any case, take US 16 into the Black Hills and at the big interchange with 16A, stay right and follow 16 and NOT 16A and go to where it Tees into 385, then west to Hill City and then down to Custer.

Stop somewhere and pick up a Black Hills and Badlands Tourist map, its invaluable for getting around and virtually everyone has them to give out. I was depending on Google Maps and the first day on the way down to Rushmore from Spearfish, the Google Maps quit, they were doing a system upgrade at Google. Thankfully the roads have very good signage and I got a map at Rushmore.

FWIW and to blow my own horn a bit, HERE is my trip report on my visit to the area last August.

Charles

Or you could take 16Byp to 79 south to Hermosa, then 36W to 16A West, straight through Custer State Park to Custer. Do NOT take 16A East from here, except in the toad.