[wingnut60]

I have always jacked my trailer up with a bottle jack placed under the spring mounting plate--have done this many times. The problem with jacking on the axle is when the jack is placed too far inward on the axle housing--the closer toward the middle of the axle, the leverage increases to the point the housing can be bent. There have been times that I haven't had the clearance to place the jack under the plate and placed it just next to the plate on the axle itself.
If you have enough jack stands, it is easily possible to use those mounting plates to get the clearance to place the jack stands under the frame and have all 4 wheels off the ground at the same time--but you have to raise it enough to allow the axles to drop as far as they can before placing the stands. I wouldn't do but one side at a time if using jack stands. Those units having the 6-point leveling systems routinely raise the trailer high enough to get the wheels off the ground without damage.
Does your trailer have the auto-leveling, or just the front legs/rear stabilizers?
Joe