I understand the cost of parts and retail cost of labor and for the work you're wanting/needing done, $750/axle isn't "bad."
Sure you can buy all the parts you need for somewhere between $25 (for a couple tubes of grease, parts cleaner etc and $500 (new complete brake assemblies and bearings/seals), but you aren't planning on tearing it apart in your driveway and repairing it, so $500 (worst case) in parts marked up 100% + the better part of a days labor @$100+/hr = $1500 all in to replace everything but drums is about right.
You can have the trailer place inspect and reuse parts, but bearings are cheap and not much savings to completely clean out old bearings vs popping in new. Brakes, provided they are in good working order, I would at a minimum replace the shoes. They may not be near worn out, but I just recently had a couple linings fall off the brake shoes on my 2005 trailer. Appears most are bonded not riveted.
Basically, $1500 gets you everything new and your hands don't get greasy. From there you or the shop could choose to save some of the parts, but you're paying similar labor charges either way.
RV dealer is a rip-off.