The Pentagon is considering converting the out-of-service luxury liner SS United States into a 2,000-bed hospital ship for use with the nation's new Rapid Deployment Force.

Feasibility studies done for the Navy by the Newport News Shipbuilding and Drydock Co. indicate it would cost between $275 million and $300 million to convert the liner to a hospital ship.

The Navy estimates that a significantly cheaper alternative would be to recommission the USS Sanctuary, a 300-bed hospital ship that was used in Vietnam but was later converted to a "dependents support ship" and stationed in San Francisco to provide medical facilities for military dependents in the area.

The Navy estimates it would cost $26 million to $42 million to convert the Sanctuary back to a 300-bed hospital ship and take it out of Maritime Administration Reserve Fleet storage at James River, Va.

In early November, Deputy Defense Secretary W. Graham Claytor notified the president of United States Cruise Inc., the original owner of the SS United States, that the Defense Department planned to ask for funds in its fiscal 1982 budget to begin the conversion.

The RDF was organized early last year to deal with military contingencies in remote areas of the world such as the Persian Gulf.