Sam's Club is part of the same company as Camping World (Affinity Group, well known for fraudulent travel schemes), and their operations seem to have become more integrated recently.
If Camping World lists a $25 higher shipping cost with UPS, the actual cost will likely be much higher, as UPS may add a "brokerage fee" as well.
In a quick web search I found that RV bookstore
sells it, and their shipping
page says that only use USPS (the post office - good), but they are out of stock. Powell's Books
is also out of stock, but their shipping info
gives long times and low prices, suggesting that they use the post office as well.
The illustrations and descriptions with these listings suggest that the publication referred to as the "Good Sam" atlas is really the Trailer Life Directory
- now labelled "Good Sam" as well - but since Trailer Life is part of the same company, this doesn't open up any new source options... just another way to search for it.
Chapters/Indigo/Coles lists it (which would be a Canadian source), but not for 2013. Either they don't have it yet, or Affinity Group has restricted it to their own retail operations.