A good auto jack has a groove in it which positively locates a round axle on top of the jack, this feature does not work if you jack on the flat underside of a camper frame. Jacking on the axle only means that its more likely that you will have to get down on all fours to place the jack and get it started. This is from experience, not opinion.
btw, if you are a shade tree mechanic, a smallish bottle jack is a good thing to have. I just used mine for an assist in removing the lower ball joint taper stud from a front steering knuckle on my Jeep Liberty.
