Many thanks to all those who have posted replies. We had already tried to find the info on government web sites, but got lost in horrendous detail about importing from Japan, etc. The pointer to the correct authority was invaluable. Looks like we can enter freely either way for up to 12 months. Haven't sorted out the visa restrictions yet, but it seems the vehicle won't be the limiting factor.
We're well on top of travel to N America. There are direct fights from Auckland to Vancouver, where I have cousins so it will be a good starting point.
Buying from the factory area would make sense if we were looking at (near) new, but there are MH on Craigslist with modest miles that better fit our budget and the prices don't seem to vary much by location.