Hello sue,
I am not aware of any caravans built in the 1980's that had filters fitted from new, so perhaps if you have one it was fitted later.
If a filter has been fitted and you have flushed through with almost any type of cleaner, yet alone Milton, the filter will be useless afterwards and will need changing.
In general filters should be changed every season, and as for cleaning, remove the filter before cleaning. The Carver Crystal Mk 1 type allows you to remove the clear cap off the filter body and it can then be used to seal the filter housing whilst cleaning and flushing.
If you are only using UK sites, the benefit of a filter is minimal, and some people are quite happy just using the water unfiltered by simply using the clear cap. - save on the cost and inconvenience of having to change the filters