Although i built this rainwater system for my house, the same principals could certainly be incorporated into a skoolie, especially for boondocking.
My downspouts enter into a 3" pipe. To build the filter, i have a perforated 3" pipe inside of a 4 inch pipe. All of the water has to pass through a nylon, which works as a course filter removing debris and other particles.
First, I took a standard 3"-4" adapter, then modified it by sanding down the "lip" The adapter is only designed to fit a couple of inches onto the smaller pipe. Using the abrasive stone in my drill, i modified this adapter to slide a couple of feet onto the pipe, like so:
Notice the lip/ring at the bottom of the adapter
in this image, i have removed part of the lip
then i slid the adapeter about 2 feet on the 3" pipe
then i attached this 4" threaded adapter
then i drilled a bunch of holes in the pipe. In theory, you could calculate out the size/number of holes vs the diameter of the pipe, but i just drilled a bunch of holes without doing any math.
then i simply put a nylon sock over top of the now perforated pipe. Having 2 layers of nylon would likely filter better.
then i simply put the perforated pipe inside the larger 4" pipe and screwed it together.
Here is the top of my first flush diverter,
and finally, my filter attached to the system.