You are certainly correct that the US government has many assets that could be sold. Many assets should be sold although I do not see that national parks will ever be sold. I do not know of any part of the Constitution or case law that guarantees federal pensions. New York state has a state constitution clause that supposedly guarantees pensions. Many other states have strong legal protections for government pensions.
These legal protections are essentially worthless if the dollar collapses. The collapse of the dollar will be facilitated by ridicuolous pension demands among other demands. Insistence by many groups including government employees for protection of their benefits will be a leading cause of the dollar collapse. This positive feedback loop (inflation will be the strong generator of positive feedback) will ensure the destruction of your pension (and large parts of the economy).
It's interesting.