The downside of poly is mostly for larger installs like a house that gets a touch of shade on a panel. That small spot of shade will cause the entire install to put out less.
Mono doesn't have that issue.
So if you plan on parking in partial shade and have more than one panel get mono.
If you're planning on having more than one panel and parking in shade isn't in the plan, get poly. It's simple.
For vans with 2-4 panels I don't think the difference is all that drastic. So either will work just fine. Google will tell you plenty of things, but it doesn't always put it into context. There's the context.
Worked for a solar company with house and larger building installs. We would actually get written permission and cut down trees and move things such as vent stacks, that caused any shade on poly panels. It was that important to getting everything out of them. They worked fine in overcast conditions. There is actually more light when overcast, with less heat. Which is ideal for solar if either type.
Larger mono builds aren't affected in the same way.
Company always preferred poly panels if we could make certain they would be in a no shade zone. If we couldn't guarantee that, we went with mono panels.
Again, with just a few panels it won't really matter that much, if at all in practical terms.
I really hope this information helps. Every 5th thread has this back and forth on panels with everyone kinda sorta knowing something. Now you know exactly. Let's make it at least every 20th thread please lol.