I'm looking for a way to turn a raw sketch with multiple paths into a single shape resulting from the average which can be filled. The solution should be tablet friendly. I have tried combinations of the smooth/join tool but it didn't work to well. How is this normally accomplished on tablets? Thanks.
You have drawn several strokes to cover the right one. You stopped when it somehow seemed to present the shape you seeked after. Now you want "the right one extractor".
I do not believe, that a programmer somewhere has written a formula which thinks like you. Hopefully some human reasoning is still needed. So, draw it yourself. Do manual tracing in a vector drawing program.
I haven't a touch surface tablet computer, but obviously Bezier curves are still possible to draw and edit to fit. They may need some tricky fingering methods, but I bet programmers have figured a way to make it possible in a productive way. With older equipment the task is trivial. Simply click with the pen tool to get polylines. Then with anchor type conversion tool pull the handles out of the nodes. Fix with the direct selection tool.
Here's a tracing example:
Lock the reference drawing to keep it in place. Draw at first a polyline (=connected line segments) around your closed shapes and along open curves. Then pull the handles out of some nodes to get curves.
In vector drawing program you cannot effectively make complex colorings without having separate closed shapes for different fill colors or gradients. Nothing prevents you to create them as you trace.
In Illustrator you have the Shape Builder, which can combine to one closed shape an area which is surrounded by separate open curves. It can be especially useful in your case, so check it carefully!
If you planned to brush the colors in a pixel graphics program, you can do it as well. The curves can be used as masks or selections, if needed.