Actually last year when I was towing my 5th wheel over to the coast, I saw a cummins at fuel stop that had stacks and a big 5th wheel. There was black soot all over the front of the 5th wheel and the guy was telling me how much he hated the noise and the soot on his trailer from the stacks. So you'd have to have some pretty tall stacks to get over the trails these days.
California law is the stacks have to be pointed straight back, not out the side or slightly out the side like the truck in the pic.
