Have you tried an application with upholstery cleaner at all?
It's not the cheapest on the market but I found that 'Vanish' upholstery cleaner works effectively and leaves a refreshing aroma too.
When we got our"new to us" Motorhome I did all the cushions and bases
Applied upholstery shampoo with a soft brush and then immediately sucked out excess moisture and dirt of course with the upholstery tool on my Vax shampoo machine.
Left them to dry thoroughly next to the radiators in the house.
If you don't have a Vax, perhaps you could borrow one for someone?
Of course if yours are removable just take them, to the dry cleaners.