Dave,
It depends how badly contaminated the tow ball / tow coupling have become.
Starting with the tow ball - if you don't have a detachable tow ball, your fixed tow ball may be a bit rusty - so a rub over with a green scotchbrite pad will clean the rust off.
Then wipe with a clean cotton rag (old T-shirt material)
If for some reason you think the tow ball may have a bit of grease or oil contamination - wipe with a cotton rag dipped in some white spirits or brake cleaner.
Then the tow coupling - inside the tow head (of an ALKO or Winterhoff stabiliser coupling) are a pair of brake pads that grip the tow ball - the surface of these pads needs to be clean and contamination free.
Wipe the surface of the pads with a clean cotton rag (or kitchen paper) - just make sure you don't transfer any grease by using an old rag.
Take a look at the cloth - it may have some black powdery deposit - this is normal. Keep wiping until the cloth comes away clean.
Robert
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