Maybe you can rent a small camper for a couple weeks. Or just buy an older one. Even a tent camper. Just don't get one too big for your truck. There are some great campgrounds in the Bighorns above Buffalo. Forest Service sites (no hookups, pit toilets, drinking water, fire pits/grills) are around $12/night, or you can just find a level spot (something like 300 feet from a roadway) and camp for free anywhere in the National Forest.
You might get lucky with an ad in the local paper,
The Buffalo Bulletin > Front but there aren't all that many private cabins in the Bighorns. It probably wouldn't cost much to try an ad for a few weeks. It's a small weekly paper.
I was pretty sure both of those places had rustic cabins; I know the Pines does. Neither are fancy places, but I'm sure when you price a cabin for a couple weeks it sounds like it must be fancy. Trust me -- those places aren't making a killing. They all seem to scrimp to stay in business, and most don't make it. It's a short season on the mountain.