On our camper, the axles are above the leaf springs. So I can put a jack and a block of wood under the tie plate below the spring directly under the axle. I do one axle at a time, one side at a time.
If the axles were flipped and located under the leaf springs, I'd use a block of wood under the axle between the u-bolts and jack from there. If the axle wall can support the weight of the camper under the vary small surface area of the spring perch, it should be able to rest on a block of wood between the u-bolts. If it makes you feel better, cut the wood block with a radius so it makes a cradle to spread the load. The other option would be to have the wood block under the u-bolts, spanning between the two. This would simulate the same loads on the axle tube as if the axle were mounted above the axle and secured with u-bolts.