Hi Sally
I would stick with Pesos only. Speaking for Rapa Nui, it's a dual Peso/USD system but the locals much prefer Pesos. They will take dollars but the exchange rate will not be good.
I can't speak for mainland Chile, as I was only there in transit, but Rapa Nui is pretty pricey (food & drinks etc all have to be flown in) so you probably need to factor that in when you are deciding your budget and buying your currency.
One place you may need USD (if JY is not including it in your package) is for purchasing your National Park ticket on arrival on the Island. It's a flat $60 fee (I can't remember the peso equivalent but most people seemed to be paying in dollars.) Perhaps JY could confirm if your TM will be buying the tickets for your group on arrival to save you all queuing at the booth.
Not long now!
Mandy