The dealer sounds like a sleezeball, as long as the chassis is under the GVWR and the axles are under the GAWR, then the shocks should be up to the task. I do believe that they gave you the reason for the porposing though. Put a good set of properly valved Bilsteins on it and you should be good to go.
03 Silverado cclb 3500 drw, D/A, Reese 22k hitch
07 Alfa SeeYa Gold 30RL,2 slides,power everything