Often, even when they have signs saying no camping, they will allow folks to overnight for a few hours.
Just north off I-80 (not far at all) in Cheyenne, WY: There is a Rest Area/Welcome Center by the a Flying J at Exit 7 (I-25). Very nice place with room for parking & to overnight (rest) for 8 hours. Never overnighted there, but have stopped at it.
Wyoming DOT Rest AreasIt is a right turn after crossing the interstate (McDs on this side as well). Left turn for Flying J. It is on a ridge or hill overlooking the country beyond. It is set away from the hustle & bustle of the truck stop & McDs.
Overnighting Sites & Sights in the
Douglas & Wheatland Area. Includes Guernsey & Fort Laramie Area.
Guernsey State Park -------
Fort Laramie National Historic Site ------
Oregon Trail Ruts and
Register Cliff