So long as it is a 36" x 36" shower it should fit. The Landmark shower is a completely different shower - one piece vs. the two piece in the ElkRidge. The Landmark shower is a premolded fiberglass shower.
To answer your question, this one piece shower can only be put in before the walls are on the coach. It would be very hard to to fit it into an existing coach.
As far as prices, there are several aftermarket accessorizers. You would have to make contact with them.