This was last suggested in 2005 apparently - I only joined SL 3 weeks ago, but I have not seen any reference to it currently or found it in a Search.
My idea is this: not everyone wants to build, or even to rent. In fact, natural 'gypsies' like myself really want to explore Second Life by travelling around at leisure, settling in a cool new area for a while, looking around, joining in with the community there, and then moving on to pastures new. (Acknowledging of course, that our existing friends network + tp + Search would make it easy to keep in touch with the wider community also).
It's just that I could never see myself buying / building / renting a home and being permanently rooted there. So I see the need for a structure that supports mobile homes and those who travel in them.
Now, I foresee one big difficulty - that the number of prims that might be involved in a mobile home would make having the necessary 'trailer park' for them rather difficult, especially if it started to fill up. But I have thought of this too - each trailer park would have a number of 'trailers' ALREADY THERE (with living room plus bedroom say), and it would then just be a matter of 'docking' one's vehicle to an empty trailer on arrival, after paying the first week's deposit.
The vehicle itself would be a simple (!) submarine/4WD/airship suitable for all terrains, and each would be owned by a resident (though there could also be some for rental). The trailers would be 'rental only' and would have a docking area on the roof for the vehicle.
All that is needed for this (apart from enterprising and enthusiastic designers for the vehicles and trailers), is land in or near each major community that can be set aside for the trailer parks, and an adminstrative structure for paying / collecting rents.
So that's it - my proposal for a proper system for mobile homes in Second Life.