[Mikendebbie]

I think he means “crawling on the dirt under the unit” is the crawl space.
Here are some screen shots of the frame in the factory from a YouTube factory tour video.
1. They bring the frame in upside-down. You can see the spare tire, axles and level-up legs being installed.
2. They flip the frame over and feed the wires, plumbing pipes and mount the gray+black tanks
3. You can see the aluminum floor framing and floor ducts installed over the frame and floor insulation laid in.
4. The plywood is screwed down over the framing and insulation.
There is plenty of room for a mouse to move thru but not a man. 😉

Search for the video Home with the Hoopers Keystone Montana Factory Tour.
There are several Montana factory tour videos you can find.