I'm aware that I'll get some blow-back on this but here goes...
If you have a bottle jack you absolutely can place it on the axle on or between the U-bolts. Don't place it more than a few inches inboard of the U-bolts or you do risk bending the axle. For safety place a jack stand just inboard of the bottle jack but don't lower the axle onto it.
I'm not a fan of lifting anything higher than needed. For servicing wheel bearings or changing a tire lifting by the axle is fine. For suspension work you do have to lift by the frame.
By lifting on the frame you'll be hanging both axles off the ground and risk flipping a hanger which is a pain to get back. And unless you like seeing your trailer leaning over precariously you'll probably need to lift both sides. Not to mention the extreme flex of lifting only one side.
As a last resort — as in side of the road flat tire — I admit to using the leveling jacks to change a tire. Ain't recommended but it's doable.
Note that the axle intentionally has a little bit of a bow for pre-load. Don't think that it's already bent.
Here is Dexter's warranty for people saying you'll void the warranty. I'm sure they
may deny a claim if you bend the axle lifting it too far inboard but you will
not bend the axle by lifting at the U-bolts. I'll not be convinced they'll deny a claim if they see scratch marks on the axle between the U-bolts.
