Andean Roads hires out motorhomes and offers long term storage, either under cover or outside. About $60 USD a month depending on the size of your truck. They seem to sometimes have problems with the contact page on the site so there should be an alternate email address there somewhere. http://www.andeanroads.com
Also several options in Uruguay, around Montevideo, easy to travel to Buenos Aires from there if you need to. http://www.hotelsuizonuevahelvecia.com/granjahotelsuizo.htm
Uruguay issues a 12mth temporary import as well which makes life easier, Argentina will issue 8mths but you may have to argue for it. Good luck.