I understood wifi was not required for AirPlay

There's such a thing as peer-to-peer AirPlay that would for example allow you to mirror the screen of your iPhone to an Apple TV. I've personally never messed with it. As for YouTube, obviously that needs an Internet connection. Your best option is to enable your iPhone's Personal Hotspot (Settings --> Personal Hotspot). This'll allow your Apple TV to piggyback your iPhone's Internet access via your cellular data plan. Whether or not this'll provide acceptable performance for watching a movie, of handle the local peer-to-peer traffic load required to use a stereo pair or homepods for ATV4K sound, I'm not sure.

Hopefully you've got a modern/powerful iPhone and an unlimited data plan with a good network provider.

FWIW, I just found this article from last year about peer-to-peer AirPlay (I never knew it was a thing). https://www.electronicshub.org/airplay-without-wifi/#3_How_to_Set_Up_Peer-to-Peer_AirPlay

Even if your [YouTube] content is downloaded to your iOS device or Mac, it still may carry copy protection that prevents 'casting' it via AirPlay. This is true of numerous iOS apps including Netflix, Disney+ and Apple TV+.

OTOH, if your ATV4K is connected to your Personal Hotspot it can just get the content directly.

If you've set up a peer-to-peer network, you should be able to stream content from your Mac, iPhone, or iPad. However, some restricted content may not allow for mirroring of display, only for direct streaming from the provider through an Apple TV app. Your purchased, downloaded content should stream just fine.

It could be that your RV does not support AirPlay. In this link, I checked to see what devices were supported. It did not list any RVs. Unless someone else here has better advice, you may want to contact Apple directly by selecting Support at the top or bottom of this page.