If my memory doesn't fail me - you can renew your towables like trailer, boat etc for another 10 years, but only once. Then you have to take it out and in again. Can't say for sure, as my vehicle and camper are in Baja where they only require TIP for boats - not campers - and my boat is under the length that would require a TIP. So, I don't tip them :)
Another explanation for campers staying on the mainland forever is that they can be "local", i.e. legally imported or purchased from somebody who did it in the past. I understand that once imported, you can put it on cinder blocks with wheels off, and it becomes a "structure". Like a barn. Doesn't require license or registration anymore. There is a similar procedure north of the border, you declare it in DMV as non-operable and that's it.