There's actually not a whole lot of difference when it comes to Caravan, RV and Camping applications to be honest. They are different cell types and are more relevant for home grid tied roof top.
You will read that poly are better in low light conditions and mono are more efficient. But for roof top caravan's the difference is negligible.
The most important thing is to get a well-constructed roof top panel from a quality brand and get the right sizing for your application.