I have travelled the roads you mention from Murdock, SD to Denver ... there are no steep grades.
There are several long stretches in South Dakota and Nebraska where it is a long distance between towns ... so keep the tank in the upper 3/4.
Valentine (and the Niobrara River valley) is an oasis. There is a nice RV park right down on the Missouri River in Pierre, SD. There is a really nice campground (with electric hookups) about 8 miles from I-80 at Ogallala, NE on Lake Ogallala ... big spaces and large cottonwood trees
2004 Winnebago Journey 39W - 2012 Buick Enclave
Present @home Home: Oshkosh, WI
We call our rig "Ernie the Journey"