Dump GoodSam as they have a poor rep. Get Coachnet instead.
As far as the parts go, gather the info about your chassis and RV, year, make, model, etc., and then call several of these
RV Salvage Yards, and/or search on their web sites if they have one. If you're clever, you might be able to get them in a bidding war to get you the lowest price. Generally, you can trust them to send you reusable parts as they don't want bad reviews that indicate they sent you junk.
Brake parts are a critical safety item so I'm not sure they'd send you the brake shoes or pads, but the surrounding parts they might without issue. Your old shoes can be resoled if they weren't ruined. If you have pads, those should be available still as a generic truck part.
When you're checking for your chassis parts, you should try to check with Truck Parts Stores instead of Auto Parts Stores. And of course you've found out that ordering parts from Freightliner wasn't prudent. They don't make your chassis.
Finally, if you have the bad parts pulled, note that many major chassis parts like for an RV have a number embossed into the metal...can't hurt to just put that number into google and see where it takes you.