I don't know if it's free but it may at least be cheap...
www.stanyanpark.com/
it's next to the park and just down the hill from St Mary's

The Laurel Inn, on Presidio And California has reasonable rates, recently renovated through joiedevivre hotels, but I don't know the parking charge. It's in a nice mini neighborhood good for waling and one short bus ride to St Mary's and the park.

http://www.jdvhotels.com/laurel_inn/contact


both have very nice continental breakfasts included...I don't know where Stanyan Park gets their croissants but they are awesome.