If you had it on the plate like you said they and a team of engineers can't prove how you damaged or will damage a thing. That's directly where the load already is.
You can also jack on the axle tube within about two inches without concern, or you can place a jackstand there for safety.
You can jack on the frame but it's a long way up there and once you make contact you still need to raise the unit by several inches to relieve suspension travel.
The quickest, easiest, in my opinion safest way is to jack on the spring pad like you did then place the stand right next to it.
Now-2014 Sierra 346RETS 5er BUB
Then-2002 Keystone Springdale 286RLDS TT
Nights camped in 2014-28, 2015-127, 2016-10