Here is a partial list of RV Salvage yards. Google (or Bing, if you prefer) 'rv salvage' to find more.
http://www.rvforum.net/SMF_forum/index.php?action=Parts
Roadtrek may be able to supply body parts, but I doubt if it will be any less expensive. $6000 doesn't buy a lot of body repair work these days, and parts cost vs labor cost is mostly a trade-off. Even with new parts , the old has to be removed, and the new installed and painted. Discuss with the shop parts cost vs labor cost and what the difference might be if you could get parts, either new or used.
If you are using an RV dealer for body repairs, consider going elsewhere. Prices may be a bit lower and any competent body shop can repair fiberglass and/or aluminum.