It is an unusual question, but you may be surprised at the answer.
Not just youtube
Streaming movies and youtube videos are often considered watching TV, but the question is based on not using any data. Playing on a USB stick, maybe? – That is also possible and useful for playing favorite films when parked. But the question is specifically about television.
Freeview – the digital television
Attaching a DVB-T2 (Freeview/digital television) adaptor to the USB socket of your Android headunit is possible. The DVB-T2 adaptor needs to be connected to an aerial like any other digital terrestrial television adaptor or set-top box. But with just the DVB-T2 adaptor and it’s aerial, you have digital television on your Android headunit. No data connection, wifi, or 4G is needed, and you are offered 100s of TV channels and radio stations, all in a digital format.
USB connected
Connect the USB to your headunit and the aerial to the USB, download the app for the adaptor, or find one on the Play Store. Scan for channels, and you are set to go.
We have used this adaptor in the past with good results. We particularly liked the supplied aerial that is quite compact yet still gets a good signal. Putting the aerial on the van roof gave the best results.
 adaptor to the USB socket of your Android headunit.){kind=link}