I asked this same question a month or two ago and here's the summary of the long thread:
No, seriously - that's essentially what the thread came down to.
- Don't ever jack on the frame - you can damage it. You should jack on the axle.
- Don't ever jack on the axle - you can damage it. You should jack on the frame.
I finally chose to jack on the axle as close to the u-bolts as possible. I was at home and bought/used a floor jack for it.
My reasoning was that there is 25" between the ground and the bottom of the frame on my fifth wheel. To get a jack up that high, it would have to be on a rickety pile of wood stacked 10" high. I just didn't feel confident in that.
I did try a ramp out of Lynx blocks with marginal success:
Tire Woes blog entry
It worked for the first tire that I had to change, but after that one - I tried it on the other side and my blocks kept slipping. The ramp that OldCoot made (very similar to a ready-made product) should work pretty well.