TL;DR: Get them to contact the Americas Service Centre, which they'll have to do anyway to initiate the process in the US, and the staff will either walk them through the process or say that they can't process them in the US.
Indian citizens need to get the Visitor Visa to enter Australia as a tourist, as they're not included for Electronic Travel Authority or eVisitor visas. They're also unable to apply for the Visitor Visa online.
The requirements to produce for an Australian visa include:
- Notarised copy of all pages of current passport.
- Two recent passport-sized photographs.
- Details of tourism-related bookings, and itinerary giving details of activities within Australia.
- Copies of financial documents indicating a consistent savings history, any assets, and taxation documents for three years.
- Letter from employer indicating job status and confirmation of approval of leave, or (if retired) evidence of retirement.
- Other health and character documents if required.
This first three are probably doable while on the road. The last three are probably going to be more difficult and may make it impossible for your parents to apply from overseas.
If all of these documents are able to be compiled while overseas, to apply for a visa while in the USA they will then need to contact the Americas Service Centre which (presumably) walks them through the process including getting the required documents to the local consulate.