KGTV-TV says the accident happened Wednesday evening near the Miramar air station.
Lt. Tyler Balzer says the 5-gallon bucket of cleaning solution was strapped down inside an Osprey but it somehow came loose and slid out shortly after the chopper took off from Miramar.
The bucket plunged through the roof of a closed auto shop and clipped a motorhome.
A half-dozen vehicles were splashed with the watery solution.
Balzer says the shop owners can file a claim with the Marine Corps for restitution.