A couple of idea's:
The brake rotors may measure perfectly true but have raised spots due to over heating. So simply measuring them for warpage doesn't show anything. Even turning them wont stop the issue because those overheated spots wear differently than the rest of the rotor. This is actually the most common cause of rotor induced vibration.
Also, does it have a hyd stabilizer (looks like a shock absorber laying down)? When they go bad they can cause this.
Rather than an RV shop that knows almost nothing about these kinds of matters, take it to a good tire or repair shop.