Do Americans have a right to decent standard of living?
No, Americans do not have any formal right to a decent standard of living. They have a right to have a chance at getting a decent standard of living, but they have no right to actually achieve such a standard.
It is important to remember that almost all rights that Americans have are negative rights. They are rights to have something not be done to you. For example, you have a right to be free from the government searching you for no reason.
Americans typically do not have more positive rights. They do not have the right to have a good living or a good job or a good house. They are not guaranteed good outcomes -- just that the government will leave them alone in some fundamental ways.