You can also take your VA letter to any National Forest Office and get the pass. The requirement is that the disability is permanent and limits or or more activities of daily living.
I got mine at the Tonto National Forest Office in Phoenix for free, no need to mail in or go to a national park.
While total disability is not required for the access pass, if 100% disabled vets there are many additional benefits available at the state level and they vary. One or more automobiles may be registered without the auto tax, Arizona grants free access to state parks and a discount on campground fees. The Access pass mentioned above gets discounted rates for National Forest and Park camping fees. Disabled vets may also get a property tax exemption, in Arizona there is an income limitation, that I will exeed on social security alone, so that is not much help. Ither states may offer more.
If you are a combat veteran (earned the campaign ribbon) surviving spouse may qualify for Dependent Indemnity Compensation even if death not related to service connected condition. There are income restrictions and net worth restrictions, house and car not part of net worth