The Campaign Legal Center complaint was filed Monday with the Federal Election Commission.
It also accused Zinke’s campaign of failing to report the identity of contributors.
In August, The Associated Press reported the campaign bought the 2004 motor home from Zinke’s wife for $59,000 and sold it a year later to a Zinke friend in the Montana Legislature for $25,000.
The complaint says the campaign either overpaid Zinke’s wife or gave a sweetheart deal to the buyer, state Sen. Edward Buttrey.
Zinke spokeswoman Heather Swift did not immediately return messages seeking comment.