Just stumbled upon this thread but thought I'd offer my 2 cents ...
I don't know what motorhomes you guys are buying but if it's built in North America (like the vast majority of MHs), no GST is paid. You will pay PST when you register it in Canada but you pay that for a unit you buy in Canada anyway ... You do have to be careful where you buy it, some states (e.g. FL) may want you to pay state taxes even if you are not registering there.
As far as if you pay tax on the full amount, I'd check with the DMV and Canadian Border Services but I'd actually be surprised if you don't pay PST only on the difference between your trade and the new purchase price. That being said, you're probably still much better off simply selling in Canada since you'll probably get a lot more for it then you would trading it in in the US.
oops, I'm not sure what I was thinking when I wrote above but you do pay the GST (it's the duty that is exempt for NorthAmerican vehicles). Sorry about the confusion (I still don't know why I thought it was GST