The exhaust on my bus originally came out the rear under the bumper. Last fall I routed the exhaust to come out the right-hand side just forward of the rear wheels. The benefits of this set up are :
A. I won't squash the tail pipe when driving through ditches;
B. Fewer bends in the exhaust. (although the single bend is 90° instead of multiple smaller bends);
C. Less exhaust gases hitting the trailer;
D. WAY shorter exhaust run.
I personally would not want an external vertical exhaust. They stick out too much from the side of the bus, they can be dangerously hot and in many cases they will have several more bends than the original exhaust set up.