Solar panels do produce power when cloudy. The amount varies and as others have noted, it won't be a tremendous amount.
Here's a cloudy day post
I put together back in 2015 with my last motorhome.
It's likely that you'll need to use your generator in these conditions but solar will still work (depending on conditions) to finish the charge so you don't have to run the generator all day long to get the final 10-15%.
Ditto on solar insolation research - that'll provide the facts.