Why hasn't a smart RV builder installed some sort of rain catchment system on the roof? I've seen rain collected off an awning. But a tray the size of a solar panel with a lip and an outlet that connects to the tank may be a good idea.

Awnings are basically kept clean of dirt and more importantly bird droppings - any water container on roof would only collect water in an unhygienic container.

I have tank water on my property in bush, gutter guards keep most crap out, I add chrorine to tank - all drinking water is filtered prior to use.