Map

I found a lot of maps--none of which were perfect, but here's one.

They are floating down river--at one point Huck points out that a slave wouldn't run South. They're going from somewhere in northern Missouri--down the black/green line and trying to get to the southern tip of Illinois.

Then the plan is to go up the Ohio River--the green line.

You could go to boondocks maps
to see some more resources.