Check out this link and read under the 'Access Pass' for more details about benefits for Disabled American (including Disabled Veterans)
U.S. National Park ServiceAmerica the Beautiful
It costs $10 if you mail in the below application, but free if you get it while you are visting a site.
The application below makes mention of receiving benefits from the VA which techincally could mean you would have to be at least 10% compensatible I guess; but there is a telephone number to call that I'm sure would get your question answered.
http://store.usgs.gov/pass/access_pass_application.pdf
USMCFLYR