"Motor homes, pull behind trailers, and other recreational vehicles greater than 18 feet in length are prohibited. This limitation does not apply to horse trailers" Badlands NP official website.

According to the bulletin posted on the Kiosk, 18ft is the longest RV permitted.

No problem. I run 32 foot long and 13.6 tall. Fallow me for more reviews and places that nobody goes. Crazy Marty

What Kruckle said. It's right there on the official website.