We had our MH registered in the US for about 1 1/2 years before we imported it and were charged the GST on the purchase price (although initially they wanted to have us pay GST on 'market value' which is them looking up similar models on AutoTrader.ca which came out to significantly more than we paid for it 18 months before). If you're buying new, you could probably get it appraised after 2 years and have the appraisal notarized and then might accept you only paying on the appraised value.
When I did it in Ontario, I had to pay GST but I did not have pay PST on registration because it was considered a 'transfer' (already registered in my name in the US) as opposed to a 'new registration' (which is taxed)