I'm amazed you have sagging on a new E450 with only a 25-foor Class C on it. Our 31 came with Hellwig booster leaves on the rear springs. Somebody took them off before we bought it used. Still sits nose low.
I want to raise the nose of ours, so it sits more level on the campsite. We always need a 2x8 block or two under the front wheels.
Our first Class C was 25-feet on E350 and it sagged when we bought it. Added one "repair leaf" to each rear spring pack and that moved it from low at the rear to higher than the front. Took one 2x8 block to become level on a level site.
It's good your shop will re-set the alignment. What Caster/Camber/Toe numbers do you have now?