I've been doing tons of research as my boyfriend and I will also be backpacking around in South America. Anyways, if you're doing lots of camping then its best to bring your own sleeping bag... you can get some that squish down pretty small and easily fit into a sleeping back compartment on your pack with room to spare.
What my boyfriend and I are doing however is we're just bringing sleeping bag liners- they're super small and thin (silk ones are best apparently). We're not sure how much camping we're doing, but that way we will reduce bed bugs/mites on us from bags we hire, and also gives us something to sleep in at hostels, and of course- is so light weight.
Best of luck- when are you hitting up South A?