Hi Jaya,
I went to Ecuador in 2013, and the US$ is widely accepted there and was in fact was the only currency that I took. This is the same in many South American countries, in that US$ are widely accepted.
A couple of the countries you are thinking of, do have a local currency so it may be a good idea to take sufficient for remote areas if you wish. However, currency can be exchanged in resort, and your tour manager will be happy to advise during the tour.
I hope this helps.
Kindest regards,
Monty