Our manual says the same thing. However, after consulting our RVIA Master Certified Tech (Todd), and a GDRV tech who worked on our RV recently, it's fine (and easier) to jack under the axle as long as you put the jack all the way out near the U-bolt. BUT, pay attention to the leaf springs. If they are getting compressed and you're getting a lot of resistance, use the frame. Another option (this tip came directly from the tech who worked on our RV) is to use the leveling system to just take a little pressure off the side on which you're working. Don't lift the whole set of tires off the ground.. just take a little pressure off and use the jack under the axle near the U-Bolt.
Our manual says to only jack up the RV using the frame, but you did it under the axle. Why?