The auto buying program is awesome... I just used it about a year ago and save a couple grand off the "bottom line, employee price, we're losing money" blah, blah blah.
From what I've been able to learn so far the RV program is not near as great. I signed up for it and ended up getting routed to a Camping World here in Oregon. I chatted with the guy and he basically told me the website price was better than Costco and that's all he could do.
I also had a buddy who just purchased and tried the Costco thing. He got further into it in that he actually went into the dealership but had the same results. No savings off their "web special".
I'm not sure what the climate is like up in your neck of the woods but down here RV dealerships are busy. It's a busy time of year and sales are quick. I don't think dealers are really motivated.
My understanding of the whole Costco thing is that the participating dealers get checked up on and better be truly offering lower prices. From what I've seen that's not the case.
If/when you explore it let us know how it comes out.