16' walls would be a minimum, I'd think, to get clearance if you have an overhead door. You may want to make them tall enough to be able to at least crawl/scoot along the roof of an RV with it in the garage for maintenance, etc. work; as with anything, it's a question of cost vs. convenience. If you ever have need of, say, replacing a rubber or TPO roof, being able to work under cover is a tremendous help.
For septic, I would see if it was possible to install a "clean out" somewhere outside that's somewhat accessible to the RV. If it's a lot of work to tie in the pole barn I would not worry about that unless you're thinking of using the RV as a guest room in the pole barn much. (Note the words "clean out" rather than "dump station" or "RV connection"; the latter often need a lot of environmental board approvals, engineering studies, and so forth, and possibly enlarging the existing septic system, while the latter is simply an addition to make maintaining the system easier. If you aren't dumping your RV tanks on a regular basis it shouldn't tax the capacity of your septic system.)
For water, I would suggest if possible setting up the run from the pressure tank (which I assume is likely in the basement of your house) to the pole barn such that it can be drained by gravity. It requires a bit of planning, and may not be practical due to the terrain, but if you can it makes it a whole lot easier to winterize the connection and obviates the need of operating heat tape through the winter and/or of burying the water line many feet below the surface of the ground, below the frost line.