If it is a crystal type I padded the rubber handles of a pair of pliars and then put a mole wrench on the working end and pushed in hard and turned the wrench very firmly and slowly in the end.
It worked.
They are meant to have something like olive oil or "something" put on the thread before you screw them in!
The original Whale inline filter fitted to our Pageant Magenta (similar kitchen layout to the Vermont) was impossible to change without removing the kitchen sink to undo the jubilee clips.
I've now fitted a Filtapac Econoflow which just needs new internals,
I converted ours to use the pushfit couplings used for domestic water taps. You can get small diameter pushfits as they are used in the supply on filtered domestic water