In the town of Guthrie, OK there is a small RV park behind the Best Western motel just off (west, then immediate south) the exit ramp. Make sure you drive PAST (do not turn into) the motel to get to it. Watch out for the narrow drive opening into the RV spot if you're pulling something.
Makes for a very nice place to start or end the day along that stretch of I35. Also makes for a good place to wait for good weather in tornado alley. Free breakfast and takes most hotel discounts (AAA)
Google Map LinkYou may have to move it around some, and zoom out.