Posing the question for anyone with experience. Looking for a way to bring my media library with me while traveling in my travel trailer. Is it possible to cast from my phone to the new Chromecast, without internet, say using my phone as a Wifi hotspot?
so you would be using your phone's hotspot to cast?
I do this all the time. Some apps allow you to cast directly from you phone, while it is the hotspot for the chromecast.
Others make you hotspot to a laptop / Chromebook and then cast from the laptop / Chromebook.
As for a personal media library, I have not done that - but in theory it may work - but would still use data from your phone.
You don't need a third device. Just use this app to create a hotspot and connect the tv (manually edit the proxy as mentioned in the app) and you can use Chromecast without any issues.
This app is also useless if you want to extend you wifi range by using your phone as a wifi repeater.
https://play.google.com/store/apps/details?id=kha.prog.mikrotik
You probably need a third device
Android TV, Hotspot. And say a tablet
That will work.
I've seen reports that what you are trying won't work