Online advice like that does tend to be pretty one-size-fits-all, which can be annoying if your circumstances are different. Still, I'm sure it wouldn't be the first time you've adapted generic advice to make it work for van life!
For YouTube PT, I like the "Ask Dr. Jo" videos. She is relentlessly cheerful and kind of goofy, which may or may not be any one person's preferred style, but she does it well, and her advice seems solid as far as I can tell.
Here's one video of hers on balance exercises.
I spot-checked the first few and they don't seem to require anything but a place to stand upright and something you can grab for support if necessary. That's probably something all balance exercises will require. If some exercises require more, you can skip those.
I don't know if she gets to the one where you just stand upright and walk heel to toe. I found that both challenging and helpful, also something where you may see relatively rapid progress once your brain figures out what you're asking it to do.
Always have something you can grab onto if you start to fall. If a chair or counter isn't practical, it seems like carrying a walking stick might work (?).
In search of a stable wall or seating, don't rule out outside walls, public buildings, picnic tables, and the like.
