Looked on Rock Auto and couldn't find the trailing arms, just the bushings...yes, may of these parts are stamped steel with pressed in bushings and it's just cheaper to replace the whole part than try to press in new bushings.
For the OP,
page 5-104 of your owners manual limits tongue weight to 10% of tow rating, so for a 3500 tow rating they say may max tongue weight is 350. So even though you were under the trailer weight rating, you may have had too much weight on the hitch and that caused the rear trailing arms to deform.
Wouldn't hurt to get an opinion at another dealer, but you will have a hard time arguing this point. From the mfr's perspective, if you were truly within all ratings, the trailing arms wouldn't have bent....